—Why is Peter working so hard these days?.

—The _______ of failing the exam next week makes him do so.

A．announcement

B．threat

C．competence

D．dilemma

当受压元件的温度低于其温度极限时，会由于温度波动变化而形成交变的热应力，在其反复作用下产生（）裂纹。

A、热疲劳

B、应力腐蚀

C、低周疲劳

D、过载