【单选题】The question set by the teacher was so difficult that the pupils did not know how to it.
【单选题】The question was so ________ that there was a ________ expression on the teacher's face.
【单选题】The question was so difficult that the pupils didn't know how to __it.
【单选题】The question was so difficult that () people could answer it.
【单选题】The question was so unexpected that I was ________for a moment.