Answer: The punctuation mark and the conjunction that can be used to combine these two sentences are a comma and "and" after "snow", that is to say these sentences can be combined by writing "snow, and it".
Explanation: When two clauses that can stand alone as sentences are joined, they must be connected with a coordinating conjunction or a semicolon. As "The precipitation that falls on the South Pole is always in the form of snow" and "It usually is the byproduct of cyclones" would act as independent clauses when combined, they need to be joined by means of the coordinating conjunction "and". Moreover, punctuation rules state that a comma must always be added before a coordinating conjunction joining two clauses. Therefore, the correct way of combining the two sentences provided is writing "snow, and it".
