Read the following article and mark the letter A, B, C, or D to indicate the correct option that best fits the numbered blanks.

Mars is millions of kilometres away but actually, the distance between Mars and the Earth (17) __________ all the time – from over 50 to 401 million kilometres. For that reason, it’s difficult to calculate the length of time it takes to get there. The fastest spacecraft can travel (18) __________58,000 kilometres per hour, so theoretically, we should get to Mars in thirty-nine days. However, this is not possible (19) __________the distance is never short for a long time. Also, you can’t travel in a (19) __________line. So it takes much longer! In 1969, Mariner 6 took 155 days to get to Mars, but Viking 2 took twice that time – 333 days!

Mars is millions of kilometres away but actually, the distance between Mars and the Earth (17) __________ all the time – from over 50 to 401 million kilometres.