--- Shall we go out for a walk?--- Sorry. This is not the right _____ to invite me. I am not in the mood to walk now.
Mary _______ her mother with the housework, but she didn’t.
The local government announced that only when the fire was under control ________ to return to their homes.
He left home without saying a word, and _______ worries many people and is to be found out.
He came all the way to China for learning it better _______ for making money.
Evidence gained from observation and experiment is often used to _______ a scientific theory.