What statement is correct about a controlled experiment

An experiment in which an observer tests a hypothesis by looking for changes brought on by alterations to a variable. In a controlled experiment, an independent variable is the only factor that is allowed to be adjusted, with the dependent variable as the factor that the independent variable will affect.

a and c are the answeres

the earth is thought to have been formed about 4.6 billion years ago by collisions from a cloud of hydrogen, helium, rock, ice and other materials. these were pulled together by gravity to form objects in and around earth like- asteroids and small early planets called planetesimals.

so, the best possible answer to this question will be option d- to harden materials to form different parts of earth’s layers