汽轮机调节系统的液压反馈是通过（）来实现的。
参考答案：改变油口面积
△ABC中，∠BCA=90°，∠BAC=60°，BC=4．在CA延长线上取点D，使AD=AB，则D，B两点之间的距离等于______．
—Do you know Huang Ting well?
—Yes. She and I _____ friends since we met in Wuhan last summer.[ ]
A. have made
B. were
C. have been
D. become
