（ ）courses enable students to extend their studies in other areas so as to widen their knowledge. (option)
A、
Option
B、
Options
C、
Optionally
D、
Optional
【正确答案】：D
【题目解析】：
修饰名词courses 需要用形容词形式，optional意为选修的。
（ ）courses enable students to extend their studies in other areas so as to widen their knowledge. (option)
（ ）courses enable students to extend their studies in other areas so as to widen their knowledge. (option)
Option
Options
Optionally
Optional
修饰名词courses 需要用形容词形式，optional意为选修的。