answer:Look at the chemical formula for each substance, and count the number of symbols in the formula. Then, decide if the substance is an elementary substance. The chemical formula for sulfur dioxide contains two symbols: S for sulfur and O for oxygen. So, sulfur dioxide is made of two chemical elements bonded together. Substances that are made of two or more chemical elements bonded together are compounds. So, sulfur dioxide is a compound, not an elementary substance. The chemical formula for radon contains one symbol: Rn. So, radon is made of one chemical element. Substances that are made of one chemical element are elementary substances. So, radon is an elementary substance. The chemical formula for chloromethanol contains four symbols: C for carbon, H for hydrogen, Cl for chlorine, and O for oxygen. So, chloromethanol is made of four chemical elements bonded together. Substances that are made of two or more chemical elements bonded together are compounds. So, chloromethanol is a compound, not an elementary substance.

answer:There is a sentence fragment that does not express a complete thought. We called a taxi for Mr. Wells. Whose car was towed from the no-parking zone in front of the firehouse. Here is one way to fix the sentence fragment: We called a taxi for Mr. Wells, whose car was towed from the no-parking zone in front of the firehouse.

answer:The second answer choice contains a vague pronoun reference. The pronoun they is used without its antecedent. The first answer choice shows a possible correction for the vague pronoun reference. They has been replaced with the instructions. Do the instructions say what these screws are used for?

answer:The source of the allusion Luddite is British history. In the early nineteenth century, factories were replacing the jobs of craftsmen. Some of these craftsmen banded together to destroy the new machinery; those who did so were called Luddites. The allusion Luddite means a person opposed to new technology.