Respuesta :
Letter C because the second sentence is a complete sentence and if you're adding the ";" then you don't need an and after it.
Answer: C. Compasses do not work at the magnetic poles; the magnetic field at these poles is vertical.
Explanation: The second sentence adds information that explains what is stated in the first sentence, therefore it is ideal to combine them with a semicolon. Combining them with "and" would defeat the purpose of having the second sentence explain the first, and combining them with a comma would be grammatically wrong since commas cannot coordinate independent clauses.