Which one of the following pairs of chemical substances is correctly categorised?

Explanation

Skeletal muscle fibres occur in bundles and are normally attached to the skeleton. Each muscle fibre is an elongated cell surrounded externally by a delicate membrane, the sarcolemma. Just beneath the sarcolemma in each fibre many nuclei occur at irregular intervals. Thus, these fibres are mutlinucleated or syncytial in nature. The cytoplasm of each fibre (sarcoplasm) has a large number of myofibrils which are tightly packed. Each myofibril shows dark bands (A bands) containing myosin and light band (I bands) containing actin, alternating with each other. That is why these are named as striped muscle fibres. Actin filaments are thinner as compared to the myosin filaments. Each actin filament is made of two 'F' actins helically wound to each other. Each 'F' actin is a polymer of monomeric 'G' (globular) actins. Two filaments of another protein, tropomyosin also run close to the 'F' actins throughout its length. A complex protein troponin is distributed at regular intervals on the tropomyosin. In the resting state a subunit of troponin masks the active binding sites for myosin on the actin filaments. Each myosin filament is also a polymerised protein made up of many monomeric proteins called meromyosins. A. Calcitonin is secreted from thyroid gland while thymosin from thymus gland. B. Pepsin is an enzyme secreted in stomach but prolactin is a hormone released from pituitary gland. D. Secretin is a hormone released from duodenum and rhodopsin is a light sensitive protein in retina.