The is the main organ of the circulatory system. what is it ? stomach, systemic circulation, or heart ?

Which prefix mean under less or below

Why did the federal government end its support of yucca mountain as a site for long-term storage of nuclear water

Which of the following is a reason why early living organisms on earth could not have survived on the surface? 1. the lack of an ozone layer 2. the lack of oxygen in the atmosphere 3. the fact that these organisms were single-celled