The socio-economic benefits of education are many; most nations require mandatory schooling.
A.documentary
B.commentary
C.factory
D.obligatory
参考答案:D
解析:[分析] 词义辨析mandatory 命令的,强制的;A.documentary 文件的;B.commentary 注释,解说词; C.factory 工厂,制造厂;D.obligatory 义不容辞的,必须的;因此D为答案。