初中英语完形一道小问题.

初中英语完形一道小问题.
Mr.Robinson had to travel somewhere on business,and as he was in a hurry,he decided to go by air.He liked sitting beside a window when he was flying,so when he got on to the plane,he looked for a window seat.He found all of them had already had been taken except for one.There was a soldier sitting in the seat beside this one,and Mr.Robinson was surprised that he had not taken the one by the window；but,anyhow,he at once went towards it.
　　When he reached it,however,he saw that there was a notice on it.It was written in ink and said,“This seat is preserved for proper load balance,thank you.” Mr Robinson had never seen such an unusual notice in a plane before,but he thought that the plane must be carrying something particularly heavy in it,so he walked on and found another empty seat,not beside a window,【to sit in.】
　　Two or three people tried to sit in the window seat beside the soldier,but they too read the notice and went on,when the plane was nearly full,a very beautiful girl hurried into the plane.The soldier,who was watching the passengers coming in,quickly took the notice off the seat beside himself and in this way succeededin having the company of the girl during the whole trip.
我想问的是在我画的那个【to sit in 】前如果不加逗号,那么是应该用以下哪一个?
A to sit B to be sat C to sit on D to be sat on