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Verbal·Standard English Conventions·Form, Structure, and Sense

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Like almost every country in the world, Britain has been allocated an international dialing prefix to help route incoming international calls. First assigned to Britain in the 1960s Red Book, a directory published by a United Nations telecommunications agency, __________

Which choice completes the text so that it conforms to the conventions of Standard English?

A

international direct dialing was not widely available until the 1970s, so initially, only switchboard operators used the prefix.

B

not until the 1970s did international direct dialing become widely available, meaning that, initially, only switchboard operators used the prefix.

C

initially, only switchboard operators used the prefix, as international direct dialing was not widely available until the 1970s.

D

the prefix was used by only switchboard operators initially, as international direct dialing was not widely available until the 1970s.