The Ancient Greek Astronomer Hipparchus Introduced the Magnitude Scale on Which

The ancient Greek astronomer Hipparchus introduced the magnitude scale on which he called the brightest stars "first magnitude". Today, the brightest star in the night sky is Sirius, with a magnitude of -1.4, considerably brighter than first magnitude. Why the discrepancy?

A) Sirius was formed since the era in which Hipparchus lived.
B) Sirius existed during Hipparchus's lifetime, but it has obviously brightened considerably since then.
C) Hipparchus had poor eyesight and made many classification errors.
D) After using modern scientific instruments to measure the actual energy output of stars, astronomers modified the magnitude scale of Hipparchus.