—Did you remember to give Marry the money you owed her？

—Yes, I gave it to her _______ I saw her.

A．while    B．the moment  C．suddenly    D．once 

这题选的是B。但我在字典里查到once也可以表示一……就 相当于as soon as 

例句如下： Once inside her apartment she felt an urge to brush her teeth.

一回到自己的公寓，她就有一种想要刷牙的冲动。

Once we were through the gates, she went off and left me on my own.

我们一进大门，她就走了，留下我一个人。

解析：the moment,强调动作A发生的时候，动作B立即发生，相当于"as soon as" 引导时间状语从句

(1)once  conj.一旦…（就…); 一…（就…） (英语解释：If something happens once another thing has happened, it happens immediately afterwards.） 

The decision had taken about 10 seconds once he'd read a market research study.在他看过一份市场调研报告后大概10秒钟就作出了决定。

Once customers come to rely on these systems they almost never take their business elsewhere.

顾客一旦依赖上这些系统，几乎就不会光顾其他商家。

Once inside her apartment she felt an urge to brush her teeth. 

一回到自己的公寓，她就有一种想要刷牙的冲动。

(2)一…就；此刻，那时(英语解释：If you say that something happens the moment something else happens, you are emphasizing that it happens immediately after the other thing.) 

The moment I closed my eyes, I fell asleep. 

我闭上眼睛就睡着了。

I recognized her the moment I saw her. 

我一眼就认出了她。 

The moment I saw you, I knew you were angry with me. 

我一看到你, 就知道你在生我的气。 

once 也可以纯粹表示时间：from the time when something happens，在这个意义上，once 等于as soon as。这时，它没有条件的含义，不影响主句动作的发生，其实，主句的动作已经发生。如果主句含有will/would 或其他情态动词表示将来或可能的意义，那once 就带有条件的味道了。

所以，两个答案都对。用作连词的the moment/the minute/the instant = as soon as = once

Once I saw his back, I knew he was not a good person. 

我一看见他那后脸儿，就知道他不是什么好人。

Once I thought of this, I also became energetic. 

一想到这些,我就来了精神。

Once he became rich, he threw aside his old friends. 

他一发财就不再理会他的老朋友了。

以上3句换成as soon as，意思保持不变。