His arms were sunburnt because he in the sun all day.
A. had sat
B. sat
C. is sitting
D. had been sitting
网友回答
A. had sat
B. sat
C. is sitting
(答案→)D. had been sitting
解析：考查时态。本句中的all day一整天，该短语与完成进行时连用，根据前半句的were说明只有过去完成进行时的形式，句意：他的胳膊赛坏了，因为一整天他都坐在阳光下。故D正确。