Why Can Astronomers Not Measure the Diameters of Stars Directly

Why can astronomers not measure the diameters of stars directly?

A) stars are so bright, their light burns out all the delicate instruments we would use to measure their diameters
B) all stars change their diameters regularly, growing alternately larger and smaller
C) stars are so far away, we cannot resolve (distinguish) their diameters
D) stars are all in binary systems, and we can only see the combined diameter of both stars
E) you can't fool me; measuring the diameter of any star is a relatively easy process