Which Has Been the Pattern of Strike Activity in the U.S

Which has been the pattern of strike activity in the U.S. over the past half-century?

A) The number of strikes has steadily increased.
B) There were peaks of strike activity in the decades of the 1950's and 1970's, but the number of strikes has been declining since 1980.
C) Strike activity was low during the decade of the 1950's, but there were peaks of strike activity in the decades of the 1960's and 1990's.
D) The amount of strike activity varies little from year to year.