Bank failures in the U.S

a. occurred frequently through the 1960s and declined since then.
b. occurred infrequently through the 1960s and have become more common since then.
c. occurred frequently through the 1930s, declined after that time, and became more common in 2008.
d. occurred infrequently through the 1930s, increased after that time, and became less common in 2008.

c