Emission scenarios, combined with modeling of the carbon cycle, has been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future.A. Emission scenarios, combined with modeling of the carbon cycle, has been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future. --- '
has been' is SV error B. Emission scenarios, combined with modeling of the carbon cycle, have been used to produce estimates of how atmospheric concentrations of greenhouse gases might change in the future.---
keeps the original intent intact and the best choice. C. Emission scenarios, combined with modeling of the carbon cycle, have been used to produce estimates of atmospheric concentrations of greenhouse gases which might change in the future. --
1. The intent is to estimate how the concentrations might change in the future. It is not to estimate the concentrations themselves 2. the use of which without the comma before is ungrammatical D. Emission scenarios, combined with the carbon cycle models, have been used for producing estimates of how atmospheric concentrations of greenhouse gases might change in the future. ---
For producing is a wrong idiom to denote intent. E. Emission scenarios, combined with modeling of the carbon cycle, have been used to produce estimates of atmospheric concentrations of greenhouse gases being changed in the future.--
1 to produce estimates of atmospheric concentrations-- is altered intent. 2. Use of 'being changed' in the future is juvenile
P.S. The introductory part - Emission scenarios, - is common to all the five choices. Why did the author underline that? Is it to frighten the test takers?