Option b is correct..
Caves are formed by the dissolution of limestone. Rainwater picks up carbon dioxide from the air and as it percolates through the soil, which turns into a weak acid. This slowly dissolves out the limestone along the joints, bedding planes and fractures, some of which become enlarged enough to form caves.
B. They are incorporated into organic material and common found in living things.<span />
N the brain<span>, dopamine functions as a </span>neurotransmitter<span>—a chemical released by </span>neurons<span> (nerve cells) to send signals to other nerve cells. The brain includes several distinct </span>dopamine pathways<span>, one of which plays a major role in </span>reward-motivated behavior<span>. Most types of rewards increase the level of dopamine in the brain, and many </span>addictive<span> drugs increase dopamine neuronal activity. Other brain dopamine pathways are involved in </span>motor control<span> and in controlling the release of various hormones. These pathways and </span>cell groups<span> form a dopamine system which is </span>neuromodulatory<span>.</span>
