A Cell Placed in Pure Water Has Swelled

A cell placed in pure water has swelled. Why?
A)The cell was in an isotonic environment, so water flowed into it.
B)The cell was in a hypotonic environment, so water flowed into it.
C)The cell was in a hypertonic environment, so water flowed into it.
D)The cell was in a hypotonic environment, so water flowed out of it.
E)The cell was in a hypertonic environment, so water flowed out of it.