Find the equations that describe the circle of radius 3 centered at (4, 2, 5) that is parallel to the xy-plane. (enter your answers as a comma-separated list of equations.)

We have that the coordinates given are for a sphere.
 As we look for a circle parallel to the xy plane, then our xy coordinates are:
 (x, y) = (4, 2)
 The standard equation of the circle is:
 (x - h) ^ 2 + (y - k) ^ 2 = r ^ 2
 where h and k are the x and y coordinates of the center of the circle and r is the radius.
 We have then:
 h = 4
 k = 2
 r = 3
 The equation of the circumference is:
 (x - 4) ^ 2 + (y - 2) ^ 2 = 9
 answer
 (x - 4) ^ 2 + (y - 2) ^ 2 = 9

First of all, this is not an observational study, but an experiment, since we have two groups and one of the two groups is manipulated or treated. In an observational study the subjects are only observed.
 
Second, the students did not choose their groups - so we can't choose the last answer.

Therefore the correct answer is:
This is an experiment because a treatment was applied to a group.