The nuclear reaction equation for the decay of a nucleus of thorium-231 (Th-231) to a nucleus of protactinium-231 (Pa-231) is shown below.

\[{}_{90}^{231}{\rm{Th}} \to {}_{91}^{231}{\rm{Pa}} + {\beta ^ - } + x\]

The particle x is a/an

A. proton.
B. antineutrino.
C. neutron.
D. electron.

B