Certain courses are compulsory; others are optional required free presure voluntary25/50Choose A, B, C or D to indicate the word or phrase that is OPPOSITE in meaning to the underlined part in each of the following questions. Certain courses are compulsory; others are optional. requiredfreepressurevoluntaryGiải thíchĐáp án A A. bắt buộc B. miễn phí C. áp lực D. tự nguyện