Check my answer: Which topic is suitably limited for a research paper?
a. The history of baseball and its greatest players
b. The home run record and batting average of Lou Gehrig ★★★
c. Why Lou Gehrig was forced to resign from baseball
d. How Lou Gehrig overcame difficulties to become one of baseball's greatest players

I chose b because there isn't much to talk about (limited). Is this right? Can someone explain if it's wrong?