TOEFL Grammar - find the error

ScottsEnglishScottsEnglish Administrator Posts: 1,060 admin ✭✭✭✭✭✭✭
edited April 2016 in TOEFL Test

Find the error in the following sentence:

Many theories to explain gravity have been develop but none are completely satisfactory.
              A                                B                               C                       D

Many theories to explain gravity have been developed, but none are completely satisfactory.

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