The truck driver was walking.

60.
100 years in a century, 365 days in a year, 24 hours in a day, 60 minutes in an hour, & 60 seconds in a minute.

If answer A would be correct, then answer B ("Answer A or B") would also be correct. If answer B would be correct, then answer C ("Answer B or C") would also be correct. This leads to the conclusion that if either answer A or answer B would be the correct answer, there are at least two correct answers. This contradicts with the statement "there is only one correct answer to this question". If answer C would be correct, then there are no contradictions.

Therefore, the solution is answer C.

68

Opportunity.

Neither—they both weigh a ton.

6 years old
1+2+3+4+5+6=21

Neither. Nine and seven equal sixteen.

It's an affirmation. The riddle isn’t describing something, it’s simply telling you how many letters are in each word.
What contains four letters. Sometimes contains nine letters.
Never contains five letters.

The children were instructed to ALL raise their hands whenever a question was asked. It did not matter whether they knew the answer or not.

If they did not know the answer, however, they would raise their LEFT hand.

If they knew the answer, they would raise their RIGHT hand.

The class teacher would choose a different child each time, but always the ones who had their RIGHT hand raised.

Mirror.

Chin.