The correct option is A Only S1 is true.
Anabolic and catabolic activities are carried out in the body at regular intervals. Hence, the waste materials generated by these activities are also excreted at regular intervals. Hence, the statement - S1 is correct. The composition of waste materials excreted out of the body depends on the type of food we eat. For example, if a person eats more protein-rich diet, then the amount of urea increases in the urine. Hence, the statement - S2 is not correct.