This question is from MGMAT SC, V.5 chapter 11. Q 10
"The young bride, as well as her husband, were amazed BY the generosity of the wedding guests."
The obvious issue here is the subject verb agreement (was/were), but I am struggling to choose between "BY" and "because".
You have mentioned in the book that we should use BY only when referring to the actual doer of the action, not the means or instruments. Should we consider the GENEROSITY the doer? or the actual doer are guests?
Thanks