Mammal? Is this for science or?
Valves ensure blood flows toward your heart. Vein valves work to bring the blood that flowed down via arteries back up to your heart.
Living things in the environment such as plants, animals, and bacteria are biotic factors. Biotic factors also include once-living parts such as dead leaves on the forest floor. Abiotic factors are nonliving aspects of the environment such as sunlight, temperature and water. One important abiotic factor is soil.
It became clear in the 1970's
I believe It is A because fissure eruptions are common on large volcanoes and also is built up by many layers of hardened lava, tephra, pumice, and volcanic ash.
