He must have been working really hard these days. This morning I even noticed behind his

问题补充：

He must have been working really hard these days. This morning, I even noticed behind his ears.

A. grey hair

B. grey hairs

C. the grey hair

D. much grey hair

答案：

A. grey hair

(答案→)B. grey hairs

C. the grey hair

D. much grey hair

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