Question

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.