They have been skating______ 2 hours ago.
I think this problem is much more difficult than that one,______?
Mother told me sound____slower than light.
—I have no idea____Mary will go shopping with us or not. –I don’t know,either.
Many people watched CCTV’s Dictation Assembly of Chinese Characters(汉字听写大会). It had a great ____on developing our native language.
____is one of the four great inventions of ancient China.