Proline Residues Are Infrequently Found in _____ Due to Their $\alpha$

Proline residues are infrequently found in _____ due to their _____.

A) $\alpha$ helices; decreased ability to serve as hydrogen-bond donors
B) $\alpha$ helices; large positive charge that disrupts the repeating structure
C) $\beta$ sheets; decreased ability to serve as hydrogen-bond donors
D) $\beta$ sheets; large positive charge that disrupts the repeating structure
E) $\beta$ turns; decreased flexibility as an amino acid