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老師們，請問下面的句子對(duì)不對(duì)啊:

1.  A set of keys are in the bag.

2.  A set of keys is in the bag.

3.  There is a set of keys in the bag.

4.  There are a set of keys in the bag.

哪些是對(duì)的，那些是錯(cuò)的呢？謝謝您。