It would be the first option. Adenine and thymine.
heir function is to base pair with the codon on a strand of mRNA during translation. This action ensures that the correct amino acid will be added to the growing polypeptide chain. A tRNA molecule will enter the ribosome bound to an amino acid.
They were adapted to an environment and once it is taken away the plants will slowly start to die. I'm not 100% :)
