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参考答案：
参考解析：
刷刷题刷刷变学霸
举一反三
【单选题】5. In the early days letters were generally not prepaid. The person ________ the letters paid the mailman when the letter was delivered.
A.
receiving
B.
received
C.
sent
D.
sending
【单选题】6. The system was ________ though as it required the mailman to find the recipient and collect the money, or if the recipient refused the letter, it had to be sent back to the originating post office ...
A.
inefficient
B.
ineffective
C.
efficient
D.
effective
【单选题】7. It was not until the early 1940s that stamps were first ________ to the Postal system.
A.
invented
B.
inclined
C.
introduced
D.
induced