Suppose a Cepheid Variable Has Been Identified, and Its Period

Multiple Choice

Suppose a Cepheid variable has been identified, and its period of brightness variability is measured to be 30 days. Its apparent magnitude is +23.3. How far away is it? (See Figure 19-21 of Universe, 11th ed.) Note: The Sun's apparent magnitude is -26.7.
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A) 3.1 × 10⁴ pc
B) 4.8 × 10⁶ pc
C) 6.0 × 10⁷ pc
D) 8.7 × 10⁹ pc