Fill in the Blank with One of the Words or Phrases

Fill in the blank with one of the words or phrases listed below. $\begin{array}{llll}\text { mixed number } & \text { equivalent } & 0 & \text { undefined } \\\text { composite number } & \text { improper fraction } & \text { simplest form } & \text { prime factorization } \\\text { prime number } & \text { proper fraction } & \text { numerator } & \text { denominator }\\\text { reciprocals } & \text { cross products }\end{array}$
-Two numbers are ___________of each other if their product is 1.

A) reciprocals
B) undefined
C) composite number
D) mixed number