50%
Assuming that the coins are fair and each have a 50% chance of
head or tails there are 4 equally likely combinations that can occur:
first coin is a head and second is a head, first a head and second a
tail, first a tail and second a head, first a tail and second a tail.
Each is as likely, 25%. The four choices are HH, HT, TH, TT, and you
can see that two of them (HT, TH) fulfill having one head and one tail,
giving you 25% +25% making is 50%.
An example where space would be a limiting factor would be this: Wolves live in a very good habitat for them - there are plenty of animals to be found and eaten. However, because their current habitat is rather limited, they cannot expand beyond a certain size because otherwise they would interfere with the habitat of each other and also reduce the amount of necessary food in the environment.
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Answer: B. global warming</h3>
As more CO2 (carbon dioxide) is released into the atmosphere, it traps more heat energy from the sun, and slowly warms the average temperature of the earth. This is the simplified version of what global warming is. Carbon dioxide is considered a greenhouse gas.
Unlike arteries, veins contain valves that ensure blood flows in only one direction. (Arteries don't require valves because pressure from the heart is so strong that blood is only able to flow in one direction.) Valves also help blood travel back to the heart against the force of gravity.
