Problem: Continue the following story.

Samantha goes to Mary to buy some supplements in powder form because 
Here's how I want to continue it: Samantha is a customer.

Problem: Continue the following story.

Rachel's research wasn't as thourough as Cynthia's because only 
Here's how I want to continue it: Cynthia had access to the chemistry lab.

Problem: Continue the following story.

Samantha was Monica's nurse while she was in the hospital emergency room, so 
Here's how I want to continue it:
Monica had her blood drawn for tests.