They have produced they did last year.

这道题目考查倍数的表示法。句子结构为"他们已经生产了是去年的两倍粮食"，表示倍数的常用结构为 “A is … times as + 原级 + as B”。而grain是不可数名词，应该用much来修饰，因此选项A “twice as much grain as” 是正确的表达。选项B “twice more grain as”、选项C “twice as many grain as”、选项D “as twice many grain as” 都不符合语法规则或表达不准确。