An investment firm rates the bonds of AardCo Inc. as "High Credit$-$Quality", and the bonds of

IMB Inc. as "Low Credit$-$Quality", while the bonds of Deva Corp. are rated "Medium Credit$-$

Quality." Which kind of measurement is used by this firm to rate bonds?

Select one:

a$.$ Ordinal.

b$.$ Interval.

c$.$ Nominal.

d$.$ Ratio.