Like longitudinal research, cross-sectional studies can be threatened by __________.

Correct Answer: D

Rationale: Cross-sectional studies can be threatened by cohort effects because different age groups may have experienced different historical or social contexts, influencing the results. This disparity can lead to invalid comparisons between age groups, confounding the study's findings. Option A, practice effects, refer to changes in participants' behavior due to repeated testing, not a specific threat to cross-sectional studies. Participant dropout (Option B) is a threat related to attrition in longitudinal studies, not cross-sectional studies. Sequential timing (Option C) does not pose a threat to cross-sectional studies; instead, it refers to the order of tasks in a study design.

Correct Answer: C

Rationale: The germinal period lasts about two weeks and occurs during the first two weeks of pregnancy. This is the time when the fertilized egg implants into the uterus and begins to divide rapidly. It is followed by the embryonic period, which lasts from about week 3 to week 8 of pregnancy. Choice A is incorrect because the germinal period happens in the early stages of pregnancy, not the second trimester. Choice B is incorrect because the germinal period typically lasts about two weeks, not six. Choice D is incorrect because the germinal period is an early stage of pregnancy and not the final period of a 38-week pregnancy.

Correct Answer: B

Rationale: The correct answer is B: 'Helps keep the temperature of the prenatal world constant.' Amniotic fluid helps maintain a stable and warm environment for the developing organism, assisting in regulating the temperature. This fluid also acts as a cushion, protecting the fetus from physical harm. Choices A, C, and D are incorrect because while amniotic fluid provides protection and support to the developing organism, it does not directly deliver food, oxygen, nutrients, or remove waste products, nor does it produce blood cells as the organs eventually take over this function.

Correct Answer: B

Rationale: The correct answer is B: nervous system and skin. The nervous system, which includes the brain, spinal cord, and nerves, as well as the skin, develop from the ectoderm layer of cells during prenatal development. Choice A is incorrect because muscles and skeleton develop from mesoderm. Choice C is incorrect because the circulatory system and other internal organs develop from mesoderm and endoderm. Choice D is incorrect because the digestive system, lungs, urinary tract, and glands develop from endoderm.

Correct Answer: C

Rationale: During the second month of pregnancy, the heart of the developing fetus begins to develop separate chambers, forming the structure necessary for a fully functioning heart. This is a significant milestone in the prenatal development process. Choices A, B, and D are incorrect because the appearance of the umbilical cord as a primitive body stalk typically occurs around week 5-6, touch sensitivity extending to most of the body is more characteristic of the end of the second trimester, and the heartbeat is usually detectable through a stethoscope around the 18th to 20th week of pregnancy, not during the second month.