Ions must be formed from molecules before measuring

them using a mass spectrometer because

$($A$)$ ions can be manipulated by electromagnetic fields in

the mass spectrometer.

$($B$)$ ionization is necessary for the molecular species to

be stable under high vacuum.

$($C$)$ the reactivity of molecular species is reduced during

the mass analysis sequence.

$($D$)$ both $($B$)$ and $($C$).$