I had just stepped out of the bathroom and was busily drying myself with a towel I heard

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A. while
(答案→)B. when
C. since
D. after
解析：考查连词：句意：我刚刚走出浴室正在用毛巾擦干自己，这时我听见脚步声。这里是在听到脚步声的那一刻，表示时间点，用when. While通常是表示时间段的，after通常是表示在…….之后。Since后跟时间点时，一般跟现在完成时连用。选B。