A lot of language learning, _____ has been discovered, is happen

A lot of language learning, _____ has been discovered, is happening in the first year of life, so parents should talk much to their children during that period.
A、as
B、it
C、which
D、This
【正确答案】：A
【题目解析】：如果选择B或D 就出现了两个句子了。本题考查非限制性定语从句,关系代词指整个一句话Alot of language learning is happening in the first year of life。传统考法是直接把从句放在句首，我不知道这里是不是算作句首，我相信学生们也会是一头雾水吧。【as用在句首，which用在句末，正确答案应该是A】【考点】考查定语从句。