Angliholic

“I don't know what's wrong with the other two choices.”

It's not a grammar question.  It's a question about meaning and common sense.  You have to be sure you understand the meanings of since, because, and though, or you can't answer the question.

People don't normally say, "Oh look!  It's raining!  Now I'll go and work outside because it's so much more convenient to work with water pouring down on me!"

since and because have meanings that make us think that the man who keeps working outside in the rain is a fool.   He works outside [since / because] it is raining.  These words give the impression that the man wants to work in the rain.

though means that he did something contrary to what most people do.  He worked in the rain, even though people don't normally work in the rain.  He knew that he was doing something that is not usually done, but he didn't care or he didn't mind.

Compare:

Though she was hungry, she did not eat.  [makes sense; maybe she was on a diet, trying to lose weight]
Since she was hungry, she did not eat.  [total nonsense; if people are hungry they eat]
Because she was hungry, she did not eat.  [same meaning as previous example; nonsense]

Isabel laughed because she was so sad.  [nonsense]
Since he had no money, Paul spent $1200.  [nonsense]
Though it was raining, the streets were all wet.  [nonsense; of course the streets will be wet if it's raining]

Note that even the nonsense examples above are grammatically correct.

CJ