C
句意：
既然已經(jīng)制定了計劃,現(xiàn)在所要做的就是怎樣執(zhí)行它.
分析：
A項been多余；
B項只能做謂語,用于句子或從句中.但本題逗號前面不是從句,因為它前后沒有相關(guān)的連詞.
D項錯在用了主動式,而本題中的plan與made是動賓關(guān)系,也就是被動關(guān)系.
the plan having been made可以轉(zhuǎn)換成從句：
Now that the plan had been made.
或者這樣轉(zhuǎn)換：
The plan had been made,so what is to be done now is how to carry it out.
不明白的再問喲!請記得采納,多謝!