The nuclear equation is incomplete. 94 to the 239th power Pu plus 0 to the 1th power n rightarrow 40 to the 100th power Zr plus ? plus 2 sub 0 to the 1th power n What particle completes the equation?

Conserve mass number and atomic number on both sides. The left side has mass number 239 + 1 = 240 and atomic number 94; after accounting for zirconium-100 and two neutrons, the missing particle has mass number 240 − 100 − 2 = 138 and atomic number 94 − 40 = 54. Element 54 is xenon, so the particle is xenon-138, choice D.

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