They kept trying _________ they must have known it was hopeless

The roads are crowded _____ walking is quicker than driving．

How difficult physics is! I really want to drop it !—_________you come on，you will do it better.

The astronauts have to lie ________ the floor of the spaceship to fight against the great pressure during the flight.

Just when we are going to have a holiday, my little son ________ have a headache.