Fill in the Blank with One of the Words or Phrases

Multiple Choice

Fill in the blank with one of the words or phrases listed below. $$\begin{array} { l l l } \text { index rationalizing } & \text { conjugate } & \text { principal square rootcube root midpoint } \\\text { complex numberlike radicals } & \text { radicand } & \text { imaginary unit } \quad \text { distance }\end{array}$$
-The_____ formula is $\mathrm { d } = \sqrt { \left( \mathrm { x } _ { 2 } - \mathrm { x } _ { 1 } \right) ^ { 2 } + \left( \mathrm { y } _ { 2 } - \mathrm { y } _ { 1 } \right) ^ { 2 } }$ .

A) midpoint
B) distance
C) conjugate
D) cube root