When does a protostar become a main-sequence star?

Anybody wanna ? (picture included? )

Aproduction line at v. j. sugumaran's machine shop has three stations. the first station can process a unit in 7 minutes. the second station has two identical machines, each of which can process a unit in 16 minutes (each unit only needs to be processed on one of the two machines). the third station can process a unit in 10 minutes. ▼ is the bottleneck station, with a bottleneck time of nothing minutes per unit (enter your response as a whole number).

What describes the formation of horizon b? a. forms at the surface b. features parent material c. undergoes the most change d. forms due to decomposed material i think the answer is c. undergoes the most change