function change_na(){
document.getElementById("africa").style.visibility="hidden";
document.getElementById("africa").style.display="none";
document.getElementById("oceania").style.visibility="hidden";
document.getElementById("oceania").style.display="none";
document.getElementById("europe").style.visibility="hidden";
document.getElementById("europe").style.display="none";
document.getElementById("asia").style.visibility="hidden";
document.getElementById("asia").style.display="none";
document.getElementById("southamerica").style.visibility="hidden";
document.getElementById("southamerica").style.display="none";
document.getElementById("northamerica").style.visibility="visible";
document.getElementById("northamerica").style.display="block";
} 

function change_asia(){
document.getElementById("africa").style.visibility="hidden";
document.getElementById("africa").style.display="none";
document.getElementById("oceania").style.visibility="hidden";
document.getElementById("oceania").style.display="none";
document.getElementById("europe").style.visibility="hidden";
document.getElementById("europe").style.visibility="hidden";
document.getElementById("asia").style.visibility="visible";
document.getElementById("asia").style.display="block";
document.getElementById("southamerica").style.visibility="hidden";
document.getElementById("southamerica").style.display="none";
document.getElementById("northamerica").style.visibility="hidden";
document.getElementById("northamerica").style.display="none";
}

function change_sa(){
document.getElementById("africa").style.visibility="hidden";
document.getElementById("africa").style.display="none";
document.getElementById("oceania").style.visibility="hidden";
document.getElementById("oceania").style.display="none";
document.getElementById("europe").style.visibility="hidden";
document.getElementById("europe").style.display="none";
document.getElementById("asia").style.visibility="hidden";
document.getElementById("asia").style.display="none";
document.getElementById("southamerica").style.visibility="visible";
document.getElementById("southamerica").style.display="block";
document.getElementById("northamerica").style.visibility="hidden";
document.getElementById("northamerica").style.display="none";
}

function change_africa(){
document.getElementById("africa").style.visibility="visible";
document.getElementById("africa").style.display="block";
document.getElementById("oceania").style.visibility="hidden";
document.getElementById("oceania").style.display="none";
document.getElementById("europe").style.visibility="hidden";
document.getElementById("europe").style.display="none";
document.getElementById("asia").style.visibility="hidden";
document.getElementById("asia").style.display="none";
document.getElementById("southamerica").style.visibility="hidden";
document.getElementById("southamerica").style.display="none";
document.getElementById("northamerica").style.visibility="hidden";
document.getElementById("northamerica").style.display="none";
}

function change_europe(){
document.getElementById("africa").style.visibility="hidden";
document.getElementById("africa").style.display="none";
document.getElementById("oceania").style.visibility="hidden";
document.getElementById("oceania").style.display="none";
document.getElementById("europe").style.visibility="visible";
document.getElementById("europe").style.display="block";
document.getElementById("asia").style.visibility="hidden";
document.getElementById("asia").style.display="none";
document.getElementById("southamerica").style.visibility="hidden";
document.getElementById("southamerica").style.display="none";
document.getElementById("northamerica").style.visibility="hidden";
document.getElementById("northamerica").style.display="none";
}

function change_oceania(){
document.getElementById("africa").style.visibility="hidden";
document.getElementById("africa").style.display="none";
document.getElementById("oceania").style.visibility="visible";
document.getElementById("oceania").style.display="block";
document.getElementById("europe").style.visibility="hidden";
document.getElementById("europe").style.display="none";
document.getElementById("asia").style.visibility="hidden";
document.getElementById("asia").style.display="none";
document.getElementById("southamerica").style.visibility="hidden";
document.getElementById("southamerica").style.display="none";
document.getElementById("northamerica").style.visibility="hidden";
document.getElementById("northamerica").style.display="none";
}