Thousands of years ago, several species of large mammals existed in North America. These species became extinct not long after the first human settlement of North America. Which human activity most likely contributed to the extinction of these mammals?
A: hunting

Some events on Earth happen instantly while others take thousands of years to occur. Which process takes the longest time to complete?
A: weathering of a rock to form small rocks and sand

Which cells help to destroy pathogens such as bacteria that enter the human body?
A: white blood cells

Earth is made of different layers that have varying characteristics. Which is the thickest layer?
A:
the mantle