Astronomers Use Roman Numerals to Indicate the Ionization State of a Gas

Multiple Choice

Astronomers use roman numerals to indicate the ionization state of a gas. Ionized hydrogenis indicated by which of the following symbols?

A) H II
B) $\mathrm { H } _ { 2 }$
C) H I
D) $\mathrm { H } _ { 1 } ^ { 1 }$
E) H III