Need help on some question for biology study guide !!!!

*name four different RNA nitrogen bases
*name the three different forms of RNA
*what is the most abundant form of RNA ?
there 30 question and thees ^ are the only one i didn't get need help ASAP please :)

1) adenine<----> uracil
guanine<----> cytosine

* the arrows symbolize which nitrogenous bases pair with each other

2) mRNA, tRNA, rRNA

3) most abundant is rRNA, or ribosomal RNA