The tribes living in the mississippi river valley from around 800 a. d. until the 1500s were known for
a. killing all of the buffalo in north america.
b. being friendly to american colonists.
c. building large earthen mounds.
d. sailing to places around the world.

the answer is C:  building large earthen mounds

Explanation:

Several tribes lived in the Mississippi River Valley from 800 A.D. until the 1500s. These tribes shared many common characteristics. One common characteristic was mound-building. Most of the tribes built large earthen mounds, which were the base for homes, temples, or tombs

answer would be b, a tariffs main purpose is to protect domestic production and jobs.

this null effect is caused by a weak manipulation and ineffective internal validity. in a scientific study, as the one dr. morimoto is trying to perfom, internal validity refers to the confidence placed in the cause-effect relationship and to how well an experiment is done and able to eliminate confounding variables.