--How do you learn English?
--I learn English ＿＿＿ practicing conversations with friends.
A.on B.by C.in

—You look very worried. ________ ?
—My mother suddenly got ill yesterday.

I found _____ necessary to save energy and protect the environment.

In our city, _____ middle school students want to work as a teacher in the future.

Don’t _____ today’s work for tomorrow. Today’s work must be done today.