How Would Astronomers,ancient or Modern,use a "Standard Candle

Multiple Choice

How would astronomers,ancient or modern,use a "standard candle"?

A) They would use its faint light to illuminate the setting dials of their telescopes.
B) They would use it to measure time,using its slow but steady melting.
C) They would use it to generate a known and extremely reproducible standard spectrum for comparison with stellar spectra.
D) They would measure distance by comparing its apparent brightness with its known absolute brightness.