问题
选择题
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
答案
答案：B
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。