Final answer:
Option A is the best way to combine the sentences because it maintains the logical connection and uses the appropriate subordinating conjunction.
Step-by-step explanation:
The best way for the author to combine sentences 7 and 8 is option A: Insulated boxes can help keep warm food warm and cold food cold, although some foods need to be refrigerated or heated to kill germs that cause food poisoning.
This option maintains the logical connection between the two sentences by using the subordinating conjunction 'although', which indicates a contrast or contradiction.