单选题 The manufacturer stipulates that ______ factory workers must receive proper safety training before being assigned to the assembly line.

【正确答案】 A

【答案解析】[解析] 词汇：形容词
1．这是一道词汇题，选项均为形容词。
2．空格后出现了复数名词factory workers，选项中可修饰可数名词复数的只有A选项all。C选项不能修饰可数名词复数。