drop-in
n.旁听生
eg. After a while, the drop-in stopped appearing at all, but he would sometimes come around to give the professor advice.
过了一阵子，这位旁听生不再来了，不过有时会前来给这位教授一些建议。

drop out
(中途)退学；退出
eg. She longed to drop out of the race and make an independent life for herself.
她真想脱离这个圈子去过自由的生活。

biological mother
生母；亲生母亲
eg. If somebody approaches the tortoise, the hippo becomes aggressive, as if protecting its biological mother," Kahumbu added.
如果有人要靠近老海龟，那么小荷马就会变得很有攻击性，就像要保护自己的生母一样。

unwed
adj. 未婚的
eg. The young, unwed lady is going to have a baby this winter.
那位年轻的且未婚的女士今年冬天就要生了。

pop
vi. 突然出现
eg. The cork popped out and the Champagne gushed from the bottle.
木塞砰的一 声打开，香槟酒从瓶中溢了出来。

relent
vi. 变得心软
eg. She relented when she saw her husband bursting into tears.
看见丈夫一哭，她的心就软了。