(a) Exact (b) Measured (c) Measured (d) Measured
Work Step by Step
(a) The number of students in the laboratory has to be exact, since it is impossible to have half a student or any fraction of a person. (b) This number is clearly estimated, and the time between two events is usually measured/estimated. (c) The mass of an object is always measured/estimated. (d) The mass of cholesterol in 1 dL of blood had to be measured. (directly or indirectly).