In skeletal muscle, the area that is composed only of both thick and thin filaments is the
a. a-band
b. i-band
c. h-zone
d. sarcomere
e. synapse

A-Band

Explanation:

Sarcomere is the basic unit of skeletal muscle and shows various striations under the microscope.

Anisotropic band or A-Band is the portion of sarcomere that contains both the thick and thin filaments . A-Band appaers dark under the microscope. Thick filaments are consist of actin and thin filaments consists of myosin.

I-Band contains only thin filaments of myosin. H- zone contains only thick filaments and no thin filament. Sarcomere is the unit of a skeletal muscle. Synapse is the junction between two neurons.

Thus, the correct answer is option (A).

the oxygenated blood then leaves the lungs through pulmonary veins, which return it to the left heart, completing the pulmonary cycle. this blood then enters the left atrium, which pumps it through the mitral valve into the left ventricle. from the left ventricle, the blood passes through the aortic valve to the aorta.

deoxygenated blood leaves through the right ventricle through the pulmonary artery. from the right atrium, the blood is pumped through the tricuspid valve (or right atrioventricular valve), into the right ventricle.