Correct Answer: D

Rationale: At around 4 months of age, babies begin to develop a preference for salty taste over plain water. This preference is part of their sensory development and exploration of tastes. Introducing a variety of flavors, including salty tastes, at this stage helps broaden the baby's palate and acceptance of different foods later on. Choices A, B, and C are incorrect as babies typically do not develop a preference for salty tastes over plain water at 2 weeks, 6 weeks, or 2 months of age. These earlier stages are more focused on milk feeding and the introduction of solid foods usually begins around 4-6 months of age.

Correct Answer: B

Rationale: A major limitation of systematic observation is that it tells investigators little about the reasoning behind responses and behaviors. Systematic observation focuses more on recording observable behaviors rather than delving into the underlying reasons or thought processes driving those behaviors. Choices A, C, and D are incorrect because systematic observation can indeed provide detailed information on how participants actually behave, it doesn't necessarily underestimate the capacities of individuals who struggle with verbal expression, and it doesn't solely ignore participants with poor memories as it can capture behavior in real-time regardless of memory recall.

Correct Answer: B

Rationale: G. Stanley Hall is generally regarded as the founder of the child study movement. He established the first American research laboratory dedicated to child psychology and education, helping to pioneer the scientific study of child development. Choices A, C, and D are incorrect as they were not primarily associated with founding the child study movement. Charles Darwin is known for his contributions to the theory of evolution, Alfred Binet is famous for developing the first intelligence test, and Sigmund Freud is renowned for his work in psychoanalysis.

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: B

Rationale: Folic acid supplementation early in pregnancy is crucial as it reduces the risk of abnormalities in the developing fetus. It is specifically known to prevent issues related to the neural tube, circulatory system, and urinary tract. Vitamin A, Vitamin D, and Vitamin C do not play the same role as folic acid in preventing these specific birth defects, making them incorrect choices.