Most multicellular organisms are composed of cells with different shapes and sizes, and those cells perform different functions. what
is the best explanation for this?

Explanation:

The best explanation is that they all have specific jobs to do..heart muscles are different to digestive cells, as are lung tissues

i'd say that the moon rock is about 2.2 billion years old.

in 1859, charles darwin set out his theory of evolution by natural selection as an explanation for adaptation and speciation. he defined natural selection as the "principle by which each slight variation [of a trait], if useful, is preserved".