Tectonic plates are large, rigid pieces of Earth's outer shell that move slowly over the mantle. They make up the lithosphere, which includes the crust and the uppermost part of the mantle, and their motion is driven by convection currents beneath. Saying a tectonic plate is a part of the crust that moves slowly gets at the heart of the idea: a plate is a substantial chunk of the crust that has its own gradual motion, rather than a surface feature or a moving body of ice or water. The other options describe things that are not movable slabs of the Earth's outer shell: a river is liquid water, a sheet of ice is frozen water, and a mountain range is a raised landform built by tectonic forces but not a moving plate itself.

Chromosomes are built around DNA. The DNA molecule carries the genetic instructions that define inherited traits, and in chromosomes it is packaged with histone proteins into a condensed structure called chromatin, which allows long DNA molecules to fit inside the cell nucleus and be efficiently organized during cell division. So the material that chromosomes are made of is DNA, with proteins helping package and regulate it. RNA plays a role in reading and expressing genetic information but is not the main component of chromosomes, and lipids are not involved in chromosome makeup.

Grasping how independent, dependent, and controlled variables work is essential for designing experiments that show cause and effect. The independent variable is the factor you deliberately change to test its impact. The dependent variable is what you measure to see the result of that change. Controlled variables are factors you keep constant across all experimental conditions so they don’t influence the outcome. This distinction matters because it lets you attribute any observed change in the outcome to the manipulation of the independent variable, rather than to other varying conditions. It also supports reliable replication and helps prevent confounding, where unaccounted factors could distort the relationship you’re trying to study. The other statements swap roles or suggest all variables should be measured the same way, which ignores the purposeful manipulation and the need to control other factors.

A carbonate reacting with an acid releases carbon dioxide. When calcium carbonate meets hydrochloric acid, the carbonate is protonated and breaks down to form calcium chloride, water, and carbon dioxide, which bubbles out as a gas. The balanced idea is CaCO3 + 2 HCl → CaCl2 + CO2 + H2O. So the gas produced is carbon dioxide. Helium, hydrogen, and methane aren’t generated by this acid–carbonate reaction, since they come from different processes and reactants.