如果（2x-3y）（M）=4x²-9y²，则M表示的式子为（　　）

A．-2x+3y

B．2x-3y

C．-2xy-3y

D．2x+3y

Charles liked music very much when he was at school, but (41) he went to the university he decided to study medicine (42) music. When he passed his examinations and became a doctor, he had to work in a hospital for (43) . There he discovered that a lot of patients were happier if (44) music was played for them. When Charles got an office and began to work (45) himself, he decided to (46) his patients happy by having a tape recorder in his waiting room play beautiful music for them.
But (47) the tape recorder had been put in(安装), Charles’ nurse heard a woman complain in the crowded (48) , "Here we’re (49) waiting to see the doctor, and he’s just playing (50) piano in his office instead of doing his work. \

[A]all[B]hardly [C]just