What two things determine how long a star will live?

There is a diagram called the Hertzsrpung Russell Diagram originated in 1919 that contains star relationships. There is a line of stars going from the top left to the bottom right called the Main Sequence. This indicates or is said to be the most stable part of a star's life. The diagram is organized by colour, surface temp. and brightness. Otherwise you would know how long a star its mass. This then relates to the 3 different masses of stars, low mass (100 billion years), medium mass (100 billion years), and High mass (7 billion years).  People may argue about the years on the star mass. 
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Source: Cramming for Science Exam! 
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The answer is letter c