The Major Weakness of Charles Darwin's Theory During His Lifetime

The major weakness of Charles Darwin's theory during his lifetime was the:

A) fact that populations do not promote competition but rather promote sharing.
B) lack of a mechanism to explain how traits were passed to offspring intact when most believed inheritance worked through blending.
C) fact that, at certain points in embryonic development, diverse species all have structures known as pharyngeal gill slits.
D) lack of variation in natural populations.