Question

what is the difference between the units rem and rad?
rem is a conventional unit, and rad is an si unit.
rem is an si unit, and rad is a conventional unit.
rem is used to measure biological risk, and rad is used to measure absorbed radiation.
rem is used to measure absorbed radiation, and rad is used to measure biological risk.

Explanation:

Rem measures biological risk of radiation, rad measures absorbed radiation. Rem is conventional, rad is SI unit.

Answer:

Rem is a conventional unit, and rad is an SI unit. Rem is used to measure biological risk, and rad is used to measure absorbed radiation.