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

入境废物是指以任何贸易方式和无偿提供、捐赠等方式进入中华人民共和国境内的一切废物(含废料)。 ( )