Apple is to fruit as onion is to _________.
Can you imagine what difficulty I had _________ (解决)the math problem?
The famous actor keeps writing novels though he is in his _________ (九十几岁).
He is ready to take on new _________ (挑战) any time.
The _________ (生气) the parents are, the more scared the children will be.
The teacher advised us to read _________ (大声地) in the morning to help us remember the passages better.