Which statement explains how two causes combined to create an effect, which became the great depression?

a) high tariffs decreased imports, which led foreign investors to withdraw large amounts of money from american banks.
b) overproduction led to increased exports, which shifted investment to foreign countries and dried up the credit available to american consumers.
c)the excessive use of credit allowed consumption to increase rapidly, which stimulated production to the point that it outpaced demand.
d)buying on margin allowed american investors to send their money abroad, which resulted in a political backlash that led congress to increase tariff rates.