Gas giants are planets whose mass is dominated by gaseous envelopes rather than solid rock. The defining feature is that their atmospheres are mostly hydrogen and helium, which is why the best statement describes them as primarily made of these two light elements. This composition gives them very thick, deep atmospheres and strong gravity that can hold onto these gases, rather than a solid surface like rocky planets have. They do not have solid surfaces accessible to standing on, and they are located far from the Sun in the outer solar system, not close to it. So, the hydrogen-and-helium dominance best captures what a gas giant is.

The Moon’s motion around Earth is best described by its sidereal month, which is about 27.3 days. This is the time it takes for the Moon to return to the same position relative to distant stars. The longer ~29.5-day cycle you might have heard of is the synodic month, the cycle of the Moon’s phases, because Earth is moving around the Sun as the Moon orbits, so it takes a bit longer to catch up in the Sun-Earth-Moon alignment. So, approximately 27 days is the orbital period around Earth. The other options don’t reflect the actual orbital period.

Genetic diversity comes from the way genetic material is shuffled and combined in sexual reproduction. Gametes are produced by meiosis, which includes crossing over and independent assortment. Crossing over exchanges chromosome segments between paired homologs, creating new allele combinations. Independent assortment randomly distributes maternal and paternal chromosomes into gametes, producing many possible genetic combinations. When two gametes fuse during fertilization, the zygote gets a unique mix of genes from both parents, and even siblings end up genetically different. This mixing means offspring are not identical to their parents or to each other, which is the essence of diversity in sexually reproducing populations.

Genetic variation in offspring comes from the mixing of DNA from two parents during reproduction. This variation is created by meiosis, the special cell division that produces gametes with different allele combinations thanks to crossing over and the random assortment of chromosomes. When two gametes fuse during fertilization, the resulting zygote carries a new mix of DNA inherited from both parents, giving the next generation inherent genetic diversity. Mitosis, by contrast, is the regular cell division that creates identical somatic cells and does not introduce new allele combinations. Cellular respiration and photosynthesis are energy-related processes and do not generate inherited DNA variation.