Specialists may be useful to _____ a situation and suggest solutions.

Hurry up! The black clouds ______that it is going to rain soon.

The spacewalk performed by Chinese astronaut Zhai Zhigang on Saturday afternoon  _____a major breakthrough in China's space program

Someone who lacks perseverance is unlikely to _______ a good researcher.

Li Lei ______ English when he was in Canada. Now he can speak it freely.