Why do astronomers think that there may be other advanced civilizations?

Because space is so huge that it would be silly to think that were the only ones

So much of space is unexplored and there are possible signs of previous life on planets such as Mars

Ibelieve the answer would be a

This is called visual imagery. it is a memorization technique wherein an idea or a term is associated with an image, this in remembering ideas, especially as humans are very visual learners. this is also why visual aids a lot for memory retention.