The following sentence contains a transitional word or phrase:
A swarm of mosquitoes meanwhile had discovered the hole in the window screen.
Which revision correctly punctuates the sentence?
A swarm of mosquitoes, meanwhile had discovered the hole in the window screen.
O A swarm of mosquitoes meanwhile, had discovered the hole in the window screen.
O A swarm of mosquitoes; meanwhile, had discovered the hole in the window screen.
O A swarm of mosquitoes, meanwhile, had discovered the hole in the window screen.