let

同意你的意见。上道题由于没有上下文选A和选C是同样正确的。

A Modern English Grammar 就曾明确指出过这一点:

This house is to let/to be let.

请再看下面的实例：

This house is to be let. (DSD) 此房出租。

更详细的情形, 见 infinitive $^{7-10}$ 。

CGEL(§ 16.52)在讲述“宾语 + 不带 to 的不定式补足语”这一动词类型时, 把 let 与 have, make 归在一类, 并特别注明 make 可用于被动语态, 而 let 和 have 则不能。 They made him understand. ~ He was made to understand. They let the prisoner go home. ~ ? The prisoner was let go home.

A. S. Hornby 在 GPU 中也指出: 当 let 作“allow”解时, 不可用于被动语态, 如不可以说 The matter cannot be let to rest here。而应该说 The matter cannot be allowed to rest here. (p. 66)

因此,我们有理由建议将上面的练习答案 Jack wasn't let to go out. 改为 Jack wasn't allowed to go out.

[附注]当然也能提出一些反例：

1. The child was let (to) do it. (《新英汉词典》, p. 728) 这孩子被允许去做那件事。

2. The grass was let grow. (GCE, p. 841) 让草长了起来。

3. The grass has been let (to) grow. (LDCE) 让草长了起来。