IMO, as per the passage:
Effects of campaigns in early stages of election: A candidate can get mostly predictive outcome, and hence will concentrate on those campaigns. (a positive outcome)
The passage shifts it focus, by including
It is now becoming clear, however, that elections can be decided in the few days preceding election dayThis finding is a positive outcome of Campaigns preceding election days, but there is a buzz word
CAN BE. So, now we can even get the predictive outcome in the days preceding elections, but it does not mean that the Campaigns in early stages will not have predictive outcome.
Then, the passage mentions a
shortcoming of late campaigns, by deducing from the defeat of candidates, that the candidates who lost the election would not have spent on early campaigns, but rather on later stages (television advertisement is just a means).
So, for
Effects of campaigns in early stages of election: We are given only positive effects. (negative effects may be there but they are not mentioned in the passage)
Effects of campaigns in later stages of election: We are given positive and negative effects (explicitly) in the passage.
Between C & D:
I would say that since Option D is mentioned, it can be inferred most reasonably, but not Option C (for the reasons mentioned above).
Hi, Experts kindly validate the above reasoning:
nightblade354,
AndrewN,
GMATNinjaTwo,
GMATNinja