Complete the following sentence with a (compound) word derived from the one(s) given in bracket.
The students expressed their doubts as to the （ ） of the experiment.(necessary)
【正确答案】：NECESSITY
【题目解析】：
本题考查的是常见形容词的名词。
由the...of可知，空缺处应填名词，necessary的名词形式是necessity意为“必要性”，as to意为“关于”，后接名词或者名词性从句。
句意：学生们表达了对实验的必要性的怀疑。