Which $RNA$ has the shortest lifespan?

What is the full form of $snRNA$?

The most abundant $RNA$ found in a cell is .......

$A$ : Catalytic functions were assigned to $RNA$ molecule during evolution.
$R$ : The rate of mutation is quite fast in $RNA$.

Given below are two statements:
Statement $I$: Catalytic RNAs are called ribozymes.
Statement $II$: $RNA$ can undergo mutations,replicate,and catalyse reactions.
In the light of the above statements,select the correct option given below:

What replaces thymine in $RNA$?