Which of the following quotes best supports the answer to part a?

a
“the first mass executions took place shortly after the soviet union and nazi germany signed a treaty of “friendship and cooperation” in august 1939, and both countries invaded poland (the wehrmacht from the west and the red army from the east).” ( paragraph 19)
b
“the second wave of murders followed the staggeringly successful june 1941 invasion of the soviet union—in brazen violation of the 1939 treaty—by the wehrmacht.” ( paragraph 22)
c
“his solution was to order a crackdown that started with the arrest of eight top air force generals, and continued—during the desperate “battle of moscow”—with the execution of at least 300 red army high-ranking officers and generals.” ( paragraph 23)
d
“as the supreme commander, stalin prohibited the red army to retreat and frequently ordered the troops to fight hopeless battles against the superbly equipped khrushchev, who served in the war as a political commissar, charged in his memoirs that stalin’s conduct of the war “cost us much blood.”” ( paragraphs 25-26)