下列各句中，划线的成语使用恰当的一项是（    ） 

同学们现在正处于青春发育期，步入青春期的信号是（   ）

—Let's go to school!

 —__________！

虽然人类都起源于共同的祖先，但是现代人类在许多遗传性状上出现显著的差异，形成了四大人种，即蒙古人种（黄种）、欧洲人种（白种）、非洲人种（黑种）和澳大利亚人种（棕种），其主要原因是(    )

完形填空。
    Hello. I'm Jenny. I have a best friend.   1 name is Amy. We are in the same school. We both can sing and dance well. She is as  2 as me.
    At school, my teachers all can make 3   happy. But we must4  white blouse with the long sleeves. I am good 5   maths and English. She 6  English. She thinks it's difficult. Yesterday we7   a party at school. We enjoyed ourselves.
    At spare time, my favourite8   is winter. I like 9  snowmen in winter. But she likes eating ice creams in summer. What a pity!
    In two months' time, we are going to go to middle school. We are very busy every day. We  10   harder than now.