Last year all refuse collected by Shelbyville city services was incine...
(A) This answer choice only remarks that the recyclable refuse shall not be incinerated; it does not provide sufficient grounds to conclude that this measure will ensure that the amount of residual ash will be halved in consonance with the halving of collected truckloads for incineration. Furthermore, since the number of truckloads destined for incineration are going to be halved regardless of whether or not any recyclable refuse shall be included therein, this answer choice does not provide a link between halved truckloads for incineration and the halving of residual ash; therefore, it is an incorrect answer choice.
(B) This answer choice remarks on the decrease in residual ash disposal costs due to recycling and does not touch upon the reduction of residual ash amounts in the slightest; while recycling efforts will certainly reduce the amount of refuse destined for incineration and save on incineration costs thereby, this does not signify a halving of the amount of residual ash; as it does not provide a link between halved truckloads for incineration and the halving of residual ash, it is an incorrect answer choice.
(C) This answer choice is irrelevant as we are not concerned with how much refuse is collected or how much of it is recyclable; already, the number of truckloads destined for incineration are to be halved, and this will happen irrespective of the proportion of recyclable materials. Furthermore, if the proportion of recyclable material is high, the incinerated material will be lesser; however, that does not essentially mean that the ash per unit of refuse will also be reduced; as this answer choice does not provide a link between halved truckloads for incineration and halving of residual ash, it is an incorrect answer choice.
(D) This answer choice provides a necessary condition for the desired halving of residual ash, alongside the given information that the number of truckloads collected will also be halved; if the number of truckloads to be incinerated will be halved, and the amount of residual ash for each truckload after incineration will be no more than it was the previous year, it follows logically that the amount of residual ash produced this year will be no more than half of last year’s; as this answer choice links the halving of truckloads to the halving of residual ash, it is the correct answer choice.
(E) This answer choice is irrelevant, as it states that the amount of refuse collected this year will remain the same as last year, which does not matter, as the number of truckloads to be incinerated will be halved regardless; it does not affect the final amount of refuse to be incinerated and, consequently, the amount of residual ash in the slightest; therefore, it does not address the link between halved truckloads for incineration and the halving of residual ash, and is an incorrect answer choice.
Hence, D is the best answer choice.