——You seem to get lost. Need help?—— _______
—I’m sorry I have failed again in the math exam.— _________ ? You’ve been working hard at the subject.
—I really don’t know how to thank you enough.— .
On our way to school, suddenly it began to rain, and we hurried to take______under a big tree.
I can still remember that it is since the 1990’s, _____ we began to use cell phones,_______ the number of mobile users has greatly increased.
—I can never forget the day ___ we spent together in the summer camp.—Well, especially the day _______it suddenly began to rain in the middle of the night.