In a cross-sectional design, researchers study __________.

Correct Answer: B

Rationale: In a cross-sectional design, researchers study groups of participants differing in age at the same point in time. This design allows researchers to compare different age groups simultaneously, providing valuable insight into age-related differences without the need to track the same group over time. Choice A is incorrect because it describes a longitudinal design where the same group is followed over time. Choice C is incorrect as it involves different groups of the same age studied at different times. Choice D is incorrect as it does not capture the essence of a cross-sectional design, which involves studying groups of different ages at the same point in time.

Correct Answer: D

Rationale: If pregnancy does not occur, the corpus luteum will shrink. As a result, the lining of the uterus is discarded through the process of menstruation. This shedding of the uterine lining is what causes menstrual bleeding. Choices A, B, and C are incorrect because they do not describe the physiological response of the body when pregnancy does not occur. The corpus luteum shrinking and the subsequent shedding of the uterine lining are part of the menstrual cycle, not processes related to cell duplication, zygote multiplication, or the duration of the germinal period.

Correct Answer: C

Rationale: The statement that is true about a blastocyst is that between the seventh and ninth days, it burrows deep into the uterine lining. This process, known as implantation, is essential for the blastocyst to establish a connection with the mother's body and receive nutrients for further development. It is a critical stage in prenatal development and sets the foundation for pregnancy. Choices A, B, and D are incorrect. A blastocyst is not a single cell but rather a structure composed of many cells. It is not a structure that feeds the developing organism; instead, it implants in the uterine lining to receive nutrients. It is not a membrane that regulates temperature but plays a crucial role in the initial stages of pregnancy by implanting in the uterus.

Correct Answer: B

Rationale: The force of blood flowing through the umbilical cord keeps it firm. The umbilical cord is the connection between the placenta and the fetus, providing essential nutrients and oxygen to the developing baby. Blood flow through the umbilical cord helps maintain its structure and function throughout the prenatal period. Choices A, C, and D are incorrect as they do not play a role in providing nutrients and oxygen to the developing baby or in maintaining its firmness.

Correct Answer: B

Rationale: The correct answer is B. During prenatal development, the ectoderm folds over to form the neural tube, which eventually develops into the brain and spinal cord. The neural tube is the precursor to the central nervous system, including the primitive spinal cord. Choices A, C, and D are incorrect because the mesoderm does not form the neural tube, the endoderm is not involved in the formation of the central nervous system, and neuron tract is not the structure that forms the primitive spinal cord.