B

common(="usual" and ordinary; happening of found often and in many places)普通的；常见的；常常发生的；到处可见的：a common experience(普通的经历)；a common tree(常见的树木)。This bird is common throughout Europe.(这种鸟在欧洲很常见。)They can treat most of the common diseases.(他们能治疗大多数常见病。)

usual(="done," found, used or existing most of the time) 经常的，通常的，惯例的，习以为常的；强调时间习惯方面，例如：This is the usual state of the house.(这房子里平常就是这种样子。)

ordinary正常的，普通的，正常的；an ordinary day(平凡的一天)，ordinary people(普通人), a piece of ordinary dance music(一首普通的舞曲)。

general普通的。它与common比较，语气稍强，含有很少例外的意外。

可见本题中用common是最佳的选择。