Which sentence uses the correct punctuation?
Because I had a stomachache, I could not go to school; furthermore I could not attend my friend's birthday.
Because I had a stomachache I could not go to school; furthermore I could not attend my friend's birthday.
Because I had a stomachache, I could not go to school; furthermore, I could not attend my friend's birthday.
Because I had a stomachache, I could not go to school: furthermore, I could not attend my friend's birthday.
Because I had a stomachache, I could not go to school, furthermore, I could not attend my friend's birthday.