The traits that are unique to insects and have probably contributed to their diversity and success include A) small body size, an exoskeleton, and sexual reproduction.
B) flight, short generation times, and complex life cycles including complete metamorphosis.
C) the exoskeleton, an open circulatory system, and jointed appendages.
D) the presence of four or more pairs of legs, an exoskeleton, and a water-resistant cuticle.