安全帽必须保证帽带扣索紧，帽子与头配戴合适，保证帽芯与帽壳间留有（）。
A、宽度
B、长度
C、数量
D、缓冲距离
参考答案：D
I became a teacher in 1993. I _______ for 13 years by next summer.
A．has taught
B．will teach
C．has been teaching
D．will have been teaching
适度土地经营规模
