It takes ________half an hour _________ his homework every day.
A:he; do
B:he; to do
C:him; to do
D:him; doing
以下有关元素性质的说法不正确的是( )
与人耳能听到的声音相比,次声具有较小的( )
抛物线的部分图象如图所示,则当y<0时,x的取值范围是____.