What is a rock? What are the main components of a rock
please say

Rocks are generally solid and contain minerals. Rocks can be created during volcanic eruptions, through erosion, or changed during metamorphism. There are many different types of rocks. To geologists, a rock is a natural substance composed of solid crystals of different minerals that have been fused together into a solid lump. The minerals may or may not have been formed at the same time.

Explanation:

answer: the blood-brain barrier is a layer of specialized membranes and cell processes designed to keep undesirable substances from passing from the bloodstream into the precious tissues of the brain. while the blood-brain barrier does keep most bad things out of the central nervous system, it is not impermeable.

explanation:

equal distribution i think.