请看下面一题：
The chemistry teacher required the students _________ more attention _________ the lab clean.
A. to pay, to keep B. to paying, to keeping
C. to pay, to keeping D. paying, keeping
【分析】此题容易误选A，认为空白处应填两个不定式。其实正确答案是C，第一空填不定式，即套用 require sb to do sth (要求某人做某事)句型，而第二空则应填 to keeping，因为 pay attention to(注意)中的to 是介词，不是不定式符号，所以其后接动词时要用动名词。类似地，以下各结构中的 to 也是介词，不是不定式符号，后接动词时应用动名词，而不是动词原形：
devote one’s time to doing sth 把某人的时间用于(奉献于)做某事
be [get] used to doing sth习惯于做某事
be equal to doing sth 等于做某事，能胜任做某事
What do you say to doing sth 你认为做某事怎么样
be accustomed to doing sth 习惯于做某事
look forward to doing sth 盼望做某事
be opposed to doing sth 反对做某事
object to doing sth反对做某事
stick to doing sth 坚持做某事
get down to doing sth 开始做某事
take to doing sth 喜欢上做某事
admit to doing 承认做了某事
pay attention to doing sth 注意做某事
