Apples release ethylene gas, which speeds up the ripening of other nearby fruits.
Why it is true
If you place an apple in a paper bag with unripe bananas or avocados, the ethylene gas will cause them to ripen much faster.
Source: Food Science
Would you rather be asked first?
This is the reading side of the site, so the answer is in the sentence above — that is the whole point of it. Test yourself works the other way round: the question comes on its own, and the fact and the explanation only appear once you have answered.