题目

 --- Where is Linda? I can’t find her anywhere.    --- She ______ the flowers in the garden. A. waters       B. is watering       C. watered        D. has watered 答案：B 已知a＝(，－1)，b＝(，)，且存在实数k和t，使得x＝a＋(t2－3)b，y＝－ka＋tb，且x⊥y，试求的最值．