Mismatched conclusion. This establishes two sufficient conditions but doesn’t take a contrapositive to link the conditions together. A class not involving extensive lab work isn’t sufficient for proving anything; it’s possible a class could sometimes be held in a classroom and sometimes in a lab. The stimulus links its conditions together to properly infer its conclusion.

Mismatched conclusion. This establishes two sufficient conditions and takes a contrapositive of the second, but doesn’t use it to draw an inference, so the conclusion is nothing more than a contrapositive of a premise. The stimulus, in contrast, uses the contrapositive to draw an inference.

Mismatched conclusion. This provides no conditional relationship sufficient to infer the conclusion. It’s possible that a class could sometimes be held in a classroom and other times in a lab, so the conclusion is unsupported. The stimulus supports its conclusion with a sufficient chain.

Mismatched conclusion. This establishes two sufficient conditions but takes the wrong contrapositive. A class involving extensive lab work is sufficient for establishing that it is held in a lab, but there’s no indication that it could not also sometimes be held in a classroom, so the conclusion is unsupported. The stimulus supports its conclusion with a conditional chain.

This argument proceeds by taking a contrapositive to link sufficient conditions. By taking a contrapositive of the second rule, we can infer that any class not held in a normal classroom must involve extensive lab work, which means it must be held in a laboratory.