A short fragment of a polymer chain (three repeat units, ethyl-type end groups shown) is drawn. Sulfur vulcanisation proceeds mainly through allylic hydrogens. Count every hydrogen in the fragment that sits on an sp3 carbon directly bonded to a carbon of a C=C bond. Give the answer as an integer.

The fragment is cis-1,4-polyisoprene. Allylic sp3 carbons are the CH2 on each side of every C=C and the methyl on each double-bond carbon, counted from the structure with RDKit. Each of the three units contributes 2 + 3 + 2 = 7 allylic hydrogens (the CH2 on each side of the C=C and the methyl), with no CH2 counted twice. The total is 21.