Look at the hr diagram below with four stars labeled. which star is twice as bright as our sun ?
a
b
c
d

The correct answer is D) Supergiants.

The star that is twice as bright as our Sun is Supergiants.

As their name indicates, Supergiants are enormous stars in the galaxy. Astronomers have used their potent telescopes to give an approximation of their size and realized that their masses are like 60 or 70 times bigger than our Sun, and their bright is hundreds of thousands brighter than the Sun.  Their span of life is approximately from 15 to 50 million years.  

(D)  It allowed the Persians to create the fastest communication system of the ancient world.

Explanation: