Nhận biết (M1)

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.