题目

The hardest language to learnPeople often ask which is the most difficult language to learn, and it is not easy to answer because there are many factors (因素) to take into consideration. Firstly, in a first language the differences are unimportant as people learn their mother language naturally. A native (本国的) speaker of English, for example, will find French much easier to learn than a native speaker of Chinese, because French is similar to English, while Chinese is very different, so first language can affect (影响) learning a second language. The greater the differences between t某学校用420元钱到商场去购买“84”消毒液，经过还价，每瓶便宜0.5元，结果比用原价多买了20瓶，求原价每瓶多少元？若设原价每瓶元，则可列出方程为  【▲】A．－ = 20B．－ = 20C．－ = 0.5D．－ = 0.5