Which of the following structures does not exist as a pair? Select one: a. The kidney. b. The sphincter muscle. c. The ureter. d. The urethra.

The urethra

Explanation: All except the urethra in the give options do not exist as a pair in the body system. The urethra is defined anatomically as the tube through which urine exits the body and, in penises, is the tube through which semen is ejaculated. It is the tube that connects the urinary bladder to the urethral opening on the external genitals in both males and females.

lysosome is this part of the cell is surrounded by a very thick outer membrane to protect the rest of the cell from its strong enzymes.

sodium i think the chemical element of atomic number 11, a soft silver-white reactive metal of the alkali metal group.