单选题 At the end of every chapter in our science text, there is a group of______questions, intended for students who wish to go more deeply into the subject.
【正确答案】 A
【答案解析】解析：optimistic“乐观的”；optional“可选择的”：portative“可搬运的，可携带的”；operational“操作的，运用的”。根据题意，正确答案为A。