Now let's look at B vs C.
B) If Maria trained hard, she would win the race.
Does this prove that Maria MUST HAVE trained hard? No - just because training hard is one way to win the race doesn't mean that it's the ONLY way to win the race. For example, maybe Sue broke her leg at the starting line. Since (B) doesn't eliminate other possible paths to victory, it doesn't PROVE the conclusion.
C) Maria could beat a four-time winner ONLY IF she trained hard.
Well, we know that Maria beat Sue,
...
B) If Maria trained hard, she would win the race.
Does this prove that Maria MUST HAVE trained hard? No - just because training hard is one way to win the race doesn't mean that it's the ONLY way to win the race. For example, maybe Sue broke her leg at the starting line. Since (B) doesn't eliminate other possible paths to victory, it doesn't PROVE the conclusion.
C) Maria could beat a four-time winner ONLY IF she trained hard.
Well, we know that Maria beat Sue,
...
