Complete the following sentence with a (compound) word derived from the one given in bracket.
（ ）courses enable students to extend their studies in other areas so as to widen their knowledge. (option)
【正确答案】：OPTIONAL
【题目解析】：
本题考查常见名词的形容词形式。
由courses可知，空缺处应填形容词进行修饰，option的形容词形式是optional意为“选修的”。
句意：选修课能够使学生扩展他们的学习领域以拓宽他们的知识面。