A naturally occurring deposit of radioactive material similar in composition to the dangerous waste produced by applications of nuclear technology has been found deep within the Earth' s crust. No ill effects have been traced to this naturally occurring deposit. Since engineers have now developed a way to store manufactured nuclear waste as deeply within the Earth' s crust as the recently found natural radioactive deposit, the safe disposal of nuclear waste can now be initiated. Which one of the following, if true, most seriously weakens the argument? ['Continued production of nuclear waste at the current rate will eventually outstrip available capacity for safe storage.', 'The total amount of dangerous waste produced annually has been declining in recent years.', 'It cannot currently be determined what the geological conditions are under which the naturally occurring radioactive deposit has apparently been safely contained.', 'Not one of the current methods used to dispose of nuclear waste has proved to be completely safe.']