Once we have________some knowledge, things do not become more difficult, but comprehensible.
The teacher said that Columbus _______America in 1492.
—When Tom________,please let me know.—Mary said when Tom________,just tell her about it.
—If you like the model car, you can _____ it. I don’t want it.—Thank you very much.
My friend Jennifer, ____ help I’ve made great progress in my studies, will leave for Canada next week.
For some students, the computer serves only _____ a tool that is used to play games.