The students found that all their homework was _____than they _____ before. [ ]

A. 1ess heavier; used to get

B. 1ess heavier; were used to getting

C. 1ess heavy; used to get

D. 1ess heavy; were used to getting

感应加热表面淬火，淬硬层深度取决于电流频率；频率越低，淬硬层越浅；反之频率越高，硬层越深。