Given a scientific passage and an answer, generate a question for the given answer.
--------
Question: Passage: Chlamydia is the most common bacterial STI in the U. S. Females are more likely to develop it than males. Symptoms may include burning during urination and a discharge from the vagina or penis. Answer: chlamydia

Answer: What is the most common bacterial sti in the u. s.?

Question: Passage: Some molecules, such as proteins, are too large to pass through the plasma membrane, regardless of their concentration inside and outside the cell. Very large molecules cross the plasma membrane with a different sort of help, called vesicle transport . Vesicle transport requires energy, so it is also a form of active transport. There are two types of vesicle transport: endocytosis and exocytosis. Both types are shown in Figure below and described below. Answer: active transport

Answer: What type of transport is vesicle transport considered to be?

Question: Passage: Water can remain in a particular location for a long time. This is known as a reservoir . Reservoirs for water include oceans, glaciers, ponds, or even the atmosphere. A water molecule may pass through a reservoir very quickly or may remain for much longer. The amount of time a molecule stays in a reservoir is known as its residence time . The residence time of water in the ocean is 3,200 years. Of course, not every molecule stays that long. That number is an average for all molecules. Answer: residence time

Answer:
What is the amount of time a molecule stays in a reservoir known as?