People take naps at noon in warmer climates, ____ the heat makes work difficult in the early afternoon. 

People take naps at noon in warmer climates, ____ the heat makes work difficult in the early afternoon. 

A．which            B．what             C．where            D．when

People take naps at noon in warmer climates, ____ the heat makes work difficult in the early afternoon. 

1. A.
   which
2. B.
   what
3. C.
   where
4. D.
   when

People take naps at noon in warmer climates, ____ the heat makes work difficult in the early afternoon. 

A. which         B. what            C. where             D. when