单句改错（共5小题; 每小题1分，满分5分）
此题要求改正所给句子中的错误，每句只有一处错误，必须按下列要求改正：
该行多一个词：把多余的词用斜线（\）划掉。
该行缺一个词：在缺词处加一个漏字符号（∧），在下面写出该加的词。
该行错一个词：在错的词下划一横线，在下面写出改正后的词。
小题1: Last night, his father came back late as usually.
小题2: At the end of 16th century, about five to seven million people spoke English.
小题3:The Mekong River becomes rapids as it passes across deep valleys.
小题4:This is the reason why she explained to me just now.
小题5:The boss requested that all the work must be finished as soon as possible.
小题1:usually 改成 usual
小题2:在16th前面加the
小题3:across改成 through
小题4:why改成which, that 或去掉
小题5:must改成should /去掉must
小题1:考查词组：as usual象平常一样
小题2:特指16世纪：在16th前面加the
小题3:across是穿过…的表面， through是穿过…的内部，穿过山谷用through。
小题4:先行词是the reason，但定语从句中缺少宾语，why改成which, that 或去掉
小题5:request接从句，从句用虚拟语气，must改成should /去掉must