As we can see, light won’t bend around the corner _____ it’s made to do so with the help of the objects that can reflect light.

A. until      B. unless

答案为unless，A 为何不行？请解释一下！

▲为你解答如下：

(1) unless=if not....除非，如果..不；(表示一种条件关系概念:即从句表示为主句动作所发生的条件....具备从句的提供的条件就会出现主句所表现的结果)（从句用一般现在时态；主句使用一般将来时态）

(2) until=before.....在.....之前（表示一种时间关系概念）（主句动作在从句所表示时间之前不发生）

中文意思：正如我们看见到的，光在拐角处不会弯曲，除非在具有能反射光特性的物体的帮助下的帮助被迫这样做.

根据以上句子表示一种条件关系概念，因此选B 答案。

▲关于until, unless具体用法如下

【1】until   prep. 到…为止；在…以前 conj. 到…为止，在…以前；直到…才

(1) prep.&conj. 直至；直到；在…之前

Until 1982 he was senior lecturer in botany at Durham University. 1982年以前他一直是达勒姆大学生物系的高级讲师。

consumers who have waited until after the Christmas holiday to do that holiday shopping. 等到过完圣诞节才进行节日采购的顾客

I waited until it got dark. 我一直等到天黑。

Stir with a metal spoon until the sugar has dissolved.  用金属调羹搅拌至糖融化。

(2) prep.&conj.（与否定词连用）直到…之前，直到…为止，除非

The traffic laws don't take effect until the end of the year. 交通法要到年底才生效。

It was not until 1911 that the first of the vitamins was identified. 直到1911年才发现了第一种维生素。

The EU will not lift its sanctions until that country makes political changes. 除非该国实施政治改革，否则欧盟将不会解除制裁。

 【2】unless conj. 除非，如果不 prep. 除了，…除外

(1)除非；若不；除非在…的时候

Unless you are trying to lose weight to please yourself, it's going to be tough to keep your motivation level high. 除非减肥是为了悦已，否则很难保持积极性。

We cannot understand disease unless we understand the person who has the disease. 若不了解患者我们就不能了解疾病。

I'm not happy unless I ride or drive every day. 只有每天骑车或开车我才会开心。