"B-Class" (B級, Bī-Kyū) is the 28th chapter of the One-Punch Man manga series.
A care package is dropped by Saitama's home. Inside the the care package is a bunch of letters from various people. Almost all of letters for Genos are positive, while the majority for Saitama are negative. Genos starts to feel very angry for the lack appreciation that Saitama is given for the countless lives that he has saved. Although Saitama simply shrugs the hatred off, Genos feels deep regret for showing Saitama the letters. However, Saitama eventually receives a thank you letter. Saitama is called in by the Hero Association to congratulate him for becoming rank one in C-Class. Saitama brought into an interview to ask him whether he wants to remain in C-Class or move up to B-Class.
Saitama decides to move up to B-Class and is given a psychoanalysis test. Overhead of the interview, there are several Hero Association staff observing and discussing about Saitama. They discuss whether or not he is a sham. After a short discussion, Sweet Mask is seen as an adviser on whether or not Saitama should move up. Sweet Mask replies that they do not need him for such as trivial task as promoting someone to B-Class. Sweet Mask additionally states that he only wants a say in promoting people to Class A and higher as they tend to have a strong influence on the association's image. After the interview, Saitama goes to a nearby a food stand to eat. There he finds Mumen Rider and it is revealed that it was he who wrote the thank you letter to Saitama. The two share a smile and order mozuku.