Biology

In eye donation, the cornea is the part of the donor's eye that is typically utilized for transplantation to restore vision in recipients with corneal blindness. Other parts like the iris, lens, and retina are generally not used in standard eye donation procedures.

The AIDS virus, known as HIV, is a retrovirus that contains single-stranded RNA as its genetic material, which it uses to replicate inside host cells.

Insulin was discovered by Sir Frederick Grant Banting and Charles Herbert Best in 1921. Among the options, Banting is the most commonly credited primary discoverer, making option C correct.

The heart beats approximately 100,000 times per day. Each heartbeat causes the four heart valves (mitral, tricuspid, aortic, and pulmonary) to open and close once. Therefore, the total number of valve openings and closings per day is about 100,000 beats × 4 valves = 400,000 valve movements. However, the question asks approximately how many times each day do our heart valves open and close normally, and the options given are much lower. Since each valve opens and closes about 100,000 times per day (once per heartbeat), the closest approximate answer for a single valve is 100,000 times (option B). The original question's option C (150,000) is not the closest to the actual figure. Thus, the correct answer should be B (100,000).

Muscle fatigue occurs due to the accumulation of lactic acid in muscles during intense exercise when oxygen supply is limited, causing a burning sensation and reduced muscle performance.

During a total solar eclipse, sunlight is blocked, reducing light availability for photosynthesis, causing the rate of photosynthesis to decrease.

Vitamin K is synthesized by intestinal bacteria in the human gut, making option D the correct answer. Vitamins B1, B12, and E are not primarily produced by intestinal bacteria.

The pancreas secretes insulin, a hormone that regulates blood sugar levels. It does not secrete Vitamin A or bile juice, which are produced by other organs.

Kidney stones are primarily composed of calcium oxalate, which is the most common constituent of these stones formed in the human kidney. Other options like sodium acetate, magnesium sulphate, and calcium alone are not typical components of kidney stones.

The femur is the longest bone in the human body, located in the thigh. It is longer than the ulna, tibia, and humerus, making option C the correct answer.