Which of the following is the most important factor to Adler's success

A．A child may accept another person's underestimate of his ability.
B．He may think that he is too young to make the most of his mental faculties.
C．A person may have the idea that he is incapable of doing good work.
D．Some old people don't believe that they are capable of learning anything new.

A．make no real efforts
B．fail to go at a job
C．show a complete lack of confidence
D．All of the above.