<Li> 1443--Nuno Tristão passes Cape Blanco . </Li> <Li> 1444--Dinis Dias reaches the mouth of the Senegal River . </Li> <Li> 1446--The Portuguese reach Cape Verde and the Gambia River . </Li> <Li> 1456--Alvise Cadamosto and Diogo Gomes explored the Cape Verde Islands . </Li>

What was the leading nation in exploration in the 15th century