In the question given below, a sentence is given, divided into parts. One of the parts may contain an error. Identify the part that contains the error, and mark it as the answer. Ignore errors of punctuation. In case the sentence is correct as it is, mark option (e), that is, 'No error'. Once docked, ISRO began tests to verify if the satellites could exchange electric power, then undock and separate, followed by testing their own payloads that would  spread over two years.