It is A. uracil instead of thymine. This is because though they are practically identical, but RNA has ribose sugar, as opposed to deoxyribose sugar in DNA and RNA has a uracil base instead of thymine.
The bases of RNA are the same as those of DNA except that RNA has A, uracil instead of thymine.
The bases used in DNA are adenine (A), cytosine (C), guanine (G), and thymine (T). In RNA, the base uracil (U) replaces thymine. DNA and RNA molecules are polymers composed of long chains of nucleotides.
RNA molecules have a skeleton of alternating phosphate groups and sugar ribose, rather than the deoxyribose found in DNA. Each sugar has one of four bases bound to it: adenine (A), uracil (U), cytosine, or guanine.
RNA is composed of four nucleobases: adenine, cytosine, uracil, and guanine. Uracil is a pyrimidine that is structurally similar to thymine, another pyrimidine found in DNA. Like thymine, uracil can base pair with adenine.
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