"D. Which of the following is possible? If it is possible, give an example. If it is not possible, explain why.
4. An invalid argument, the conclusion of which is a tautology."
Rocks are dense.
Rocks are heavy.
Therefore, rocks are either rough, or they're not.
Correct answer: Not possible.
Where did I go wrong?
There was another question I wasn't too sure of, however the textbook didn't give an answer for this one. The question is:
"B. For each of the following: Is it a tautology, a contradiction, or a contingent sentence?
6. If anyone has ever crossed the Rubicon, it was Caesar."
My answer: Contradiction.
Is this correct?