请教英语高手unless和until的区别

Question

David’s parents ——him to go to the Internet café again —— his grades improve.
A. forbid; unless B. forbid; until
我觉得A和B 好像都对，到底哪个是正确答案呢？

Answer 1

首先从字面意思上理解，unless是除非，如果不，要是不，until是直到..才
然后就是一般用法的问题了，until 一般用于完成时，而此题很明显只是一个强调性的句子，所以应该选A

Answer 2

A 一般用untill的话是用"not..untill"

Answer 3

unless = if... not

unless his grades improve= if his grades doesn't improve
这样句子就通顺了。