This is a paraphrasing task. In this task, you're given a sentence and your task is to generate another sentence which express same meaning as the input using different words.
--------
Question: they did not speak again until they had reached the end of the tunnel .

Answer: they did n't speak a word until they reached the end of the corridor .

Question: it explained how a dozen robots could follow when one robot had been summoned , but only when needed and not otherwise .

Answer: it could also explain why one robot was called upon when a robot was called , a dozen - but only when they needed to be .

Question: m mass of the vehicle in kilograms ( kg )

Answer:
engine specific emissions co 2 = 175 + a ( m - m 0 ) where : m weight of the vehicle in kilograms ( kg )