如图是由5个相同的小正方体组成的立体图形,这个立体图形的俯视图是( )
A:
B:
C:
D:
小磁针静止时总是指南北,是因为地球周围存在着____ .对小磁针有____ 的作用.
No one knows how the huge rocks _______ and _______ without our modern machines eight hundred years ago.
— ______sleep do you think is necessary for a Grade 9 student?— 8 hours at least.
8-4____3
7+2____10