The lady was so absorbed in her job that she didn't hear a customer at the door.
A. to knock
B. knocking
C. be knocked
D. knocked
网友回答
A. to knock
(答案→)B. knocking
C. be knocked
D. knocked
解析：答案：B 考查非谓语动词。句意：这位女士如此专注于她的工作以至于她没有听到有位顾客正在敲门。hear sb. doing sth.听到某人正在做某事。