Correct Answer: A

Rationale: In an experimental study, the independent variable is the factor being manipulated or controlled by the researcher. In this case, the frequency of read-alouds is the variable that researchers would intentionally change or vary to see its effect on the dependent variable, which is the children's vocabulary scores. The other options (B. type of books parents read, C. child's vocabulary score, and D. difficulty of vocabulary words) are not the independent variable in this study as they are either irrelevant or are the outcome being measured.

Correct Answer: B

Rationale: Stress hormones during labor and delivery actually help arouse the infant into alertness. These hormones can promote the infant's readiness to adapt to the new environment outside the womb.

Correct Answer: C

Rationale: The yolk sac is an extraembryonic membrane that plays a significant role in the early development of the embryo. One of its functions is to produce blood cells until the organs are mature enough to take over this function. This process occurs during the early stages of prenatal development to support the growing embryo before it can sustain itself independently.

Correct Answer: B

Rationale: Newborns of smoking mothers display more muscle tension. This is because nicotine in cigarettes can lead to increased muscle tension in babies, which can affect their physical development and potentially lead to future health issues. This is one of the negative effects of smoking during pregnancy on the developing fetus.

Correct Answer: B

Rationale: Practice effects refer to the phenomenon where participants in a longitudinal study show improvement in test performance over time due to becoming familiar with the tasks or test materials. This can be considered a limitation as it may distort the results by showing an artificial improvement rather than a true developmental change.