As we know, different from the ones nowadays, the earliest locomotives were ________ by stream.
句意：正如我们所知道的，和现在不同的是，最早的机车是通过蒸汽来推进的。propel推动； 推进。compel强迫，迫使。impel推动；驱使。repel击退；抵制。故选C。