It's very important ______ us ______ hard.
A:for; to study
B:of; to studying
C:for; studying
D:of; to study
下列语句中,标点符号使用正确的一项是( )
当价格运行到a点时,可能出现的经济现象是( )
算一算。
690-200=____340+200=____ 340+20=____
5700+100=____ 8100-700=____ 5700-100=____