Comparative anatomy shows how related organisms share a common heritage that has been modified over time. When you compare the skeletons of different vertebrates, such as humans, cats, whales, and bats, you often find a similar arrangement of bones in their forelimbs. That pattern isn’t just about what the limbs do today; it reflects a shared blueprint inherited from a single ancestor. Over millions of years, each lineage tweaked that blueprint to fit its environment, but the basic bone arrangement remains, signaling a distant common origin. That’s why the best choice is that they share a common ancestor in distant evolutionary past. The other ideas don’t fit because the observed structural similarity points to relatedness, not complete independence or no relationship; and comparing species isn’t about being the same species.

Think of the nucleus as the library that holds our genome. In humans, there are about 20,000 distinct genes that code for proteins, plus additional genes for various RNAs. That makes roughly 20,000 genes contained in a nucleus when we talk about the number of different genes present. This is a standard, widely cited figure for the human genome. The other numbers are far from typical counts for human genes: 5,000 is too few, while 50,000 or 100,000 would be more than the human genome contains even when including noncoding RNA genes. If you consider that a diploid cell has two copies of each gene, you’d have two alleles for each gene, but the question usually refers to distinct genes rather than copies.

A parabolic mirror is defined by its reflecting surface being shaped like a parabola, and the key idea is how that shape handles incoming light. A parabola has the property that light rays coming in parallel to its axis reflect and converge at a single point, the focus. That means distant light is concentrated to one spot, which is why parabolic mirrors are ideal for gathering and focusing light in systems like telescopes, satellite dishes, and solar concentrators. Other shapes don’t produce this same single focus: a spherical mirror can focus parallel rays poorly due to spherical aberration; a flat mirror simply reflects without changing the convergence or divergence of the beam; a convex mirror makes parallel rays diverge, creating a smaller, virtual image instead of a real focus.

Parasitism is a relationship in which one organism benefits at the expense of another. The parasite gains nutrients, shelter, or a habitat, while the host suffers harm, such as reduced fitness or health problems. This fits the described concept because the benefit goes to one participant and the cost to the other. For contrast, mutualism involves both organisms benefiting; commensalism is when one benefits while the other is largely unaffected; and symbiosis is a broader term that covers several types of close associations, not a single one-sided harm. Examples include tapeworms in the intestines or ticks feeding on blood, illustrating the one-sided advantage that defines parasitism.