Insert the missing symbol in the following reactions.
(a) $_{2}^{4} \mathrm{He}+_{6}^{12} \mathrm{C} \rightarrow_{7}^{15} \mathrm{N}+?$
(b) $_{1}^{2} \mathrm{H}+_{1}^{3} \mathrm{H} \rightarrow_{2}^{4} \mathrm{He}+?$
(c) $^{1}_{0} \mathrm{n}+235 \mathrm{U} \rightarrow_{54}^{140} \mathrm{Xe}+?+2_{0}^{1} \mathrm{n}$
(d) $_{1}^{3} \mathrm{H} \rightarrow ?+_{-1}^{0} e$