Howard is a young entrepreneur who is invested in creating a new form of social media. He is sure of his idea and believes that it will be successful in the long run. However, he is careful not to be overoptimistic about it and works on other ideas as well. In the given scenario, which of the following statements is true of Howard?
选项：

A: He has an external locus of control.
B: He is docile.
C: He is a conformist.
D: He is confident but not overconfident regarding his abilities.