At Chernobyl, One of the Harmful Radioactive Isotopes Was ¹³¹I

Multiple Choice

At Chernobyl, one of the harmful radioactive isotopes was ¹³¹I, a beta emitter with a halflife of 8 days. Suppose some of this isotope is deposited on a field. What can we say about this field 40 days later?

A) Its radioactivity level has decreased to 1/32 of its original level.
B) Its radioactivity level has decreased to 1/64-- one sixty- forth-- of its original level.
C) Its radioactivity has decreased to harmless levels.
D) Its radioactivity level has decreased to 1/16 of its original level.
E) It is just as radioactive as it was to begin with.