When Does a Protostar Become a Main- Sequence Star

When does a protostar become a main- sequence star?

A) at the instant that the first hydrogen fusion reactions occur in the protostar's core
B) when it becomes luminous enough to emit thermal radiation
C) when a piece of a molecular cloud first begins to contract into a star
D) when the rate of hydrogen fusion becomes high enough to balance the rate at which the star radiates energy into space

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