$20.$ What is a $51\%$ attack?

A$.$ An attack where an attacker gains control of $51\%$ of the cryptocurrency on a blockchain$$network

B$.$ An attack where an attacker gains control of $51\%$ of the nodes on a blockchain network

C$.$ An attack where an attacker gains control of $51\%$ of the computing power on a blockchain network

D$.$ An attack where an attacker gains control of $51\%$ of the smart contracts on a blockchain network