____she couldn't understant was___fewer and fewer students showed interested in her lessons填what why还是 why what打错了 是understand

____she couldn't understant was___fewer and fewer students showed interested in her lessons
填what why还是 why what
打错了 是understand

what why
understand是及物动词,只能接what；后面句子是个完整的句子,陈述事实,只能用why