He came here __________.
The computer system _____ suddenly while he was searching for information on the Internet.
Cycling is highly _____ to people’s health and the environment.
I recognize her __I met her at the airport, although we haven’t seen each other for ten years.
Unless ________ to speak, you should remain silent at the conference.
_____ in Haikou for five years, he is not used to the weather in his hometown.