If you ________ home without a word, your parents wouldn't be so worried now.

A．didn't leave

B．couldn't have left

C．hadn't left

D．shouldn't leave

（）是控制叶片脱落的主要激素。这是因为它能促进细胞壁降解酶（）酶的合成，并且控制（）酶由原生质体释放到细胞壁中，从而促进细胞衰老和细胞壁的分解，迫使叶片、花或果实机械地脱离。