51. They planned to go to Beijing for sightseeing, but because of their daughter’s unexpected illness they had to stay at home _____.
Sweet memories ______ as I looked at the pictures _____were taken at the beach last summer.
Dr Yuan makes______ possible______ one-third more of the crop in the same fields.
I don't have enough money.I, _____, can't afford to buy Audi A3 Sportback.
Where was it ______ you _______ our maths teacher?
----- Mary looks very upset. She _______ failed to pass the exam. ----- I guess so. It's very difficult after all.