The scientists from the UK were the first _____ an animal __

The scientists from the UK were the first _____ an animal _______.A.to clone; successfully
The scientists from the UK were the first _____ an animal _______.
A.to clone; successfully B.cloned; successful
C.to clone; successful D.cloned; successfully

A
（我好像理解你哪里不懂了）
其实你要是把它当作固定搭配看就很容易了,记住这样的结构：
It is the first time (that)+现在完成(从句)
It was the first time (that)+过去完成(从句)
而the first to do sth本来就是是一个常理 （序数词修饰的要用不定式）
但由于上面这个从句结构,就要用完成时（就记住上面这个是特殊的吧）
但是如果做题,看到是从句结构,你就要警惕,记住要用完成时
比如：Is this the first time that you have visited Hongkong?
这是你第一次参观香港吗?
如果是普通的结构,就用不定式,比如：He is the first to come .