Read this sentence from the short story "The Man Who Was Almost a Man" and answer the question.

"When his father and brother had left the kitchen, he still sat and looked again at the guns in the catalogue, longing to muster courage enough to present his case to his mother."

What does the phrase "to muster courage" mean in this context?

A. to overcome courage
B. to summon up courage
C. to concentrate his courage
D. to make courage disappear