Answer:
The region where the embryoblast contacts the trophoblast is called the embryonic pole. ... Just before implantation, the cells in the embryoblast start to differentiate into two layers - the epiblast (primary ectoderm), and an internal layer of cuboidal cells called the hypoblast (or primary endoderm).
Skeletal muscle is attached to bones and its contraction makes possible locomotion, facial expressions, posture, and other voluntary movements of the body.
The answer is A. resonance. A resonant frequency means a natural frequency which is a part of vibration made by the physical parameters of any vibrating object. A vibrating object will get the resonant frequency from complex excitation and would vibrate.