Correct Answer: B

Rationale: By using random assignment of participants to treatment conditions, investigators increase the chances that participants' characteristics will be equally distributed across treatment groups. Random assignment helps reduce the likelihood of bias in the selection process and ensures that any differences between groups are due to the treatment and not pre-existing group differences. Sequential, systematic, and correlational assignment methods do not provide the same level of assurance in distributing participants' characteristics equally across treatment groups.

Correct Answer: A

Rationale: Lev Vygotsky viewed cognitive development as a socially mediated process, emphasizing the role of social interactions and cultural influences in shaping a child's cognitive development. This is in contrast to Jean Piaget, who focused more on individual interactions with the environment. Vygotsky believed that cognitive development is significantly influenced by social interactions with others, highlighting the importance of collaborative learning and cultural context. Choices B, C, and D are incorrect as they do not align with Vygotsky's perspective on cognitive development.

Correct Answer: C

Rationale: In a structured observation, each participant has an equal opportunity to display the behavior of interest because the researcher designs a specific set of conditions or tasks for all participants to engage in, providing a standardized environment for observation. This allows for fair and equitable observations across all participants, ensuring that each individual has the same opportunities to exhibit the behavior being studied. Choice A, clinical interview, typically involves a one-on-one interaction between a researcher and a participant focusing on collecting information through questions and answers, not providing equal opportunities for behavior display. Choice B, naturalistic observation, involves observing individuals in their natural environment without intervention or manipulation of conditions, which may not always ensure equal opportunities for behavior display. Choice D, case study, focuses on in-depth analysis of a single individual or a small group, providing detailed information but not necessarily ensuring equal opportunities for behavior display among participants.

Correct Answer: B

Rationale: The correct answer is B: 'In South Africa, 30 percent of all pregnant women are HIV-positive.' This statement is true as South Africa has a high prevalence of HIV among pregnant women, emphasizing the need for interventions to prevent mother-to-child transmission. Choices A, C, and D are incorrect. Choice A is inaccurate as women account for more than 50% of new HIV infections globally. Choice C is misleading as the majority of new HIV infections actually occur in sub-Saharan Africa, not just developing countries. Choice D is also incorrect as without treatment, the risk of mother-to-child transmission is higher than 30%.

Correct Answer: B

Rationale: A sequential design permits researchers to check if cohort effects are operating. This design involves combining cross-sectional and longitudinal research, allowing for the comparison of different cohorts at different ages. By using this approach, researchers can determine if developmental changes observed are due to age or generation-specific factors. Therefore, a sequential design is valuable in addressing the potential influence of cohort effects on developmental outcomes. Choices A, C, and D are incorrect because a sequential design does not neglect diversity in developmental outcomes, is not necessarily less efficient than a longitudinal design, and can make both cross-sectional and longitudinal comparisons.