I think the answer would be D goodluck
To prevent blood flowing back after each heartbeat. They ensure that the blood flows only in one direction (back to the heart).
The first reason was that his theory of continental drift was just too weak for most geologists to accept. Even though he believed the supercontinent that broke up into different continents moved, he did not have a clear explanation to how the continents moved. The other reason is that some of his explanation clashed with ideas that were widely accepted in the science communities. He used similar fossils from different continents to back up his theory of continental drift. However, at that time, many scientists that had observed similarities in fossils in places like South America and Africa believed there were similar fossils in different continents because of a land bridge that were formed by two continents.
Many science communities believe that land bridges allowed migration of many different species and even people to one place to another by large bodies of water frozen by low temperature known as ice age.
People, coal, and gasoline are sources of CO2
