My English is improving. It is getting _____ and _____. [ ]

Many scientists believe that the universe began with an enormous explosion called the “Big Bang”, ______ happened about 13.7 billion years ago.

They ______ on the project for almost one month before I joined them, and now we ______ on it as no good results have come out so far.

This is the reason ________ he didn’t come to the meeting[ ]

下列各项中，没有语病的一项是(　　)