A nucleoside is a ribose plus a base, with no phosphate. When one
or more phosphates are bonded to the ribose or deoxyribose, the nucleoside
becomes a nucleotide. Note that RNA and DNA are defined by the presence of
ribose or deoxyribose, respectively, and not by the bases attached to them.
However, uracil is commonly found in RNA, and thymine in DNA, and thus the
exercise will only accept those bases for attachment to the respective ribose
and deoxyribose groups.