The Shell of a Planetary Nebula Is Measured by the Doppler

Multiple Choice

The shell of a planetary nebula is measured by the Doppler shift of emission lines to be expanding outward at a speed of 10⁴ m/s,while its radius is measured to be 1 ly,or about 10¹⁶ m.Roughly how long has the shell been expanding? (Hint: 1 year = 3.15 × 10⁷ sec.)

A) 30 years
B) 30,000 years
C) 30 million years
D) 10¹² years