Which sentence correctly uses a comma to separate the introductory phrase? before researchers are granted access to the rare-book stacks, they must be approved through a security screening process. before, researchers are granted access to the rare-book stacks they must be approved through a security screening process. before researchers are granted access, to the rare-book stacks they must be approved through a security screening process. before researchers are granted access to the rare-book stacks they must be approved through, a security screening process.