How much of the Earth’s surface does the US and Canada make up? A.12%
B.16%
C.22%
D.33%

Humans have survived and thrived in mexico city by using technology to create greater access to land. when the spanish conquered the aztec capital, tenochtitlan, in the 16th century, they built mexico city by draining the lakes and filling in the lake water to create more arable land for agriculture. this technique of land reclamation has continued to be fundamental to human civilization in mexico city. 

Well, a and b together would be negative. b alone would also be negative.