Reactants would be the missing word in the sentence because they make up the products in a chemical reaction
Uranium-235 would be more useful for dating in Cambrian time because Cambrian time was 540 million years ago while the half life of carbon-14 is only 5,730 years
Hope this helps
+2, because the element has more protons than electrons now
C (s) + 2 H2 (g) = CH4 (g)
