When he was asked about the question where he was last night, he was _____[ ]

This is an unpleasant subject_ ____ we might argue for a long while.[ ]

When asked why he went there, he said he was sent there _____ for a space flight. [ ]

He doesn’t think the question of ________ they are boys or girls is important.[ ]

Subway Line 4，into use in September 2009，has made traveling in Beijing easier．