柴油机的速度特性指（）随转速变化的规律。
参考答案：有效功率、有效转矩、有效燃油消耗率
Tina, ________ for months to find a job as a waitress, finally worked as a tour guide at a local advertising agency.
A．struggling
B．struggled
C．having struggled
D．to struggle
地球上最大的生态系统叫做 ( )
A．生物圈
B．大气圈
C．水圈
D．岩石圈
