Which of the following describes some of the changes that occur during the second trimester of pregnancy?

A. The fetus doubles in mass, and the body begins to store nutrients and minerals within its tissues.
B. Cells divide into three cell layers during gastrulation, and most of the major organs become fully formed.
C. The ectoderm, mesoderm, and endoderm are made, and the heart can be heard with a stethoscope.
D. Bone starts to replace cartilage to form the skeleton, and the fetus begins to form and discharge urine.