What on earth are the police busy with?—They have been looking into how the plane crash _______.

‘What can I do?’ I thought hard. Suddenly, a good idea went ______my mind.[ ]

This math question is too difficult. Can you help me?

Pop music _________ relaxing.

If you can’t decide which of the two novels to borrow, why don’t you take_____? —- I won’t read them this month.