be to do 和 be doing 做主语

题目

be to do 和 be doing 做主语
the first thing when you go to school is( )the teacher.
a.greeting b.to greet c.greet d.greeted
选哪一个?

答案

The first thing when you go to school is( b )the teacher.
a.greeting b.to greet c.greet d.greeted
【解析】动名词和动词不定式作主语的差别是：前者表抽象概念,即一件事,而后者表达一次具体动作.
序数词往往跟动词不定式连用.