I ______ him in the park yesterday afternoon

15. I ______ him in the park yesterday afternoon.
            A. did saw                   B. seed                        C. was see                   D. saw
16. My sister ______ lots of flower on the balcony.
            A. plant                       B. planted                   C. did planted             D. was plant
17. His uncle ______ a new house in the suburb.
            A. buyed                     B. was buyed              C. did bought              D. bought
18. Where ______ they last weekend?
giải thích vì sao nha