有“绿色水库”之称的是哪种生态系统( )
A:森林生态系统
B:海洋生态系统
C:湿地生态系统
D:农田生态系统
例:
He didn't fly a kite yesterday.
He did his homework yesterday.
已知AC⊥BC,AC=BC,D满足CD→=tCA→+(1﹣t)CB→,若∠ACD=60°,则t的值为( )