帮我详细解决几个英语单选题 谢谢

1、________ that Joe saw the film"ET" directed by Steven Spielberg,he made up his mind to become a director,too.
A.When B.The very first time C.As soon as D.While
这道题正确答案是B，但里面的very是什么意思 为什么不能用when

2、The reason ______ he gave for being late was _____ he overslept
this morning.
A.why;that B.why;because C.which;taht D.which;because
这道题正确答案是c，为什么不能用why？什么情况下用why,什么情况下用which？

3、Do you know our town at all?
No,this _____ the first time I ____ here.
A.was;has come B.is;come C.is;had come D.is;have come
这道题正确答案是D，为什么要用have?

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