这是一道1996年的高考题.正确的说法是
Many people agree that a knowledge of English is a must in international trade today.
尽管knowledge是不可数名词,但有时可以加不定冠词,意为"对……有某种程度的了解."
此处must也作名词,英语解释为：Something that is absolutely required or indispensable（绝对必要或不可避免的应当去做之事）例：
Promptness on the job is a must.Comfortable shoes are a must when taking a walking tour of Wales.
工作上准时是绝对必要的.步行去威尔士旅行时舒服的鞋是绝对必要的物品