Assertion: The Soviet economy was growing rapidly by the late 1970s.
Reason: The USSR became increasingly bureaucratic and authoritarian leading to stifled growth and political weakness.

The Assertion is incorrect but the Reason is correct.

The Soviet Union lagged behind the West in technology, infrastructure (e.g., transport, power), and most importantly, in fulfilling the political or economic aspirations of citizens. Most of the institutions of the Soviet state needed reform: the one-party system represented by the Communist Party of the Soviet Union had tight control over all institutions and was unaccountable to the people. The Soviet economy was faltering in the late 1970s and became stagnant.