1.I've never seen anyone run so fast一____David
go.
A. just watch B. just to watch
C. just watching D. just having watched选哪个,为什么?属于什么结构?
2.____we can have a sports meet this afternoon
depends on ____ we have good weather.
A. If,on if B. Wether ;on if
C. That ;on it that D. Wether; on it if是C还是D?为什么/
3. My grandparents still treat me like a child. They can't imagine ____ grown up.
A. my B. mine C. myself D. me
imagine 后能否用动词的过去分词宾语补足语?

1.1.I've never seen anyone run so fast一__A__David
go.
A. just watch B. just to watch
C. just watching D. just having watched
选A.因句子的意思是“我从来没有见过有人跑的如此快—看David跑得多快”。后面应该是个祈使句。
2.._Whether___we can have a sports meet this afternoon
depends __whether__ we have good weather.
A. If,on if B. Whether ;on if
C. That ;on it that D. Whether; on it if
两个空都应填whether。句子意思是“我们今天下午能否举行运动会取决于我们能否有个好天气”。第一个空whether“是否”，引导主语从句，if不能。第二个空whether“是否”，引导宾语从句，作介词on的宾语，也不用if。
3.My grandparents still treat me like a child. They can't imagine ____ grown up.
A. my B. mine C. myself D. me
imagine的用法有：
1）imagine (sb或sb's)doing .如：I can't imagine her marrying him.
2)imagine加从句。如：Can you imagine what he has done?
3)imagine+宾语+宾补（形容词）。如：Imagine yourself (to be ) rich and famous.
你问的这个题应属于用法3），me作imagine的宾语。这儿grown up相当于形容词adult，mature。