1.People take naps at noon in hotter climates,-----the heat

1.People take naps at noon in hotter climates,_____the heat makes work difficult in the early afternoon.
A.which B.what C.when D.where
（正确答案是D.请问这道题是先行词是什么?是noon吗?climates是名词,那为什么后面用where?）