Answer: b. Retaliatory tariffs by trading partners
Step-by-step explanation:
In the 20s, the United States instituted a series of tariffs on imports that culminated with the Smoot-Hawley tariff of 1930 as they hoped to protect the local industry and to increase government revenue.
Some countries replied with their own tariffs on American exports such that American exports to these countries fell significantly and world trade reached a new low as well.