When I entered the classroom I saw Mary at the desk and a letter.A. seating; writingB.

问题补充：

When I entered the classroom, I saw Mary at the desk and a letter.

A. seating; writing

B. seated; wrote

C. sitting; wrote

D. seated; writing

答案：

A. seating; writing

B. seated; wrote

C. sitting; wrote

(答案→)D. seated; writing

解析：考查非谓语动词。第一空Mary与seat为动宾关系要用过去分词作补语，与sit为主谓关系要用现在分词作补语；第二空Mary与write为主谓关系要用现在分词作补语。故选D。句意：我走进教室看到玛丽坐在桌子旁边写信。