Answer the following questions.

Questions 11-15 are based on the following text.

    The driest places on Earth are deserts. A desert is such a dry place because rain rarely falls. Some deserts have no rain for years, and some never rain at all. Moreover, deserts are scorching hot, especially during daytime. It is so hot that it is even possible to fry an egg with a pan on top of the sand.

    Strong winds blow across deserts. This makes the sand pile up to make heaps and dunes of sand. The biggest dune that has ever existed is 200 metres tall. Sometimes, these are dangerous to humans because it can cover villages in the deserts.

    Interestingly, not all deserts are sandy, only a quarter. In fact, most deserts are covered in stones and gravel.

Not all deserts are sandy ____ most deserts are rocky.