Why Might It Be Difficult for Earth-Like Planets to Form

Why might it be difficult for Earth-like planets to form around Sun-like stars in the inner parts of the disk of our galaxy?

A) any stars would be sucked into the central supermassive black hole
B) there would be insufficient amounts of heavy elements
C) the stars would be packed together too densely for planets to be stable
D) there would be too much harmful radiation