If fossil-fuel combustion continues at its present rate, then a greenhouse effect will occur. If a greenhouse effect occurs, then world temperatures will rise. Therefore, if fossil-fuel combustion continues at its present rate, then world temperatures will rise. (F, G, W)

Translate the following arguments into symbolic form. Then determine whether each is valid or invalid by constructing a truth table for each. The letters to be used in constructing the truth tables are given in parentheses.