21. It has been claimed that ________ red wine is good for your heart.
(A) drink
(B)a drink
(C) to have been drinking
(D) drinking

drinking red wine 整句是主詞

個人判斷是：
 It has been claimed that ________ red wine is good for your heart.

(that ________ red wine is good for your heart.)  這整句是名詞子句

名詞子句結構：S+V

所以主詞應該是( ____red wine)+is (動詞)+good for your heart(主詞補語)

那這裡主詞裡面的結構考的是分詞的用法
現在分詞表示主動的概念(語態上)以及動作正在進行(時間上)→the falling leaf 正在飄落的葉子

過去分詞表示被動的概念(語態上)以及動作已經發生或完成(時間上)→the fallen leaf 以落地的葉子

(A) drink → 名詞子句結構就變成V+S+V，就算當成名詞 也很怪 
(B) a drink→翻成一杯酒+red wine看似很合理，但drink 已經是酒的總稱，不會合起來用
(C) to have been drinking→跟(A)一樣是結構問題，不贅述
(D)drinking→人喝酒是主動喝，人不會被酒喝，所以用drinking不是drank