40According to paragraph 2, a lighter or massless fuels have to be created because ______.
(A) the weight and space requirement of current fuel technology are too cumbersome
(B) they have to keep the spaceships smaller
(C) spaceships cost too much to add the mass and the space of heavy fuels
(D) the current fuel technology is one of the effective ways to make a long interstellar journey possible

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