John, an expert in game theory, predicts that negotiations cannot be r

John, an expert in game theory, predicts that negotiations cannot be resolved unless one party is willing to concede a symbolic step. He also believes that when a symbolic step of concession is taken, negotiations will be resolved. Other game theory experts, however, believe that these results do not take other variables into account.

Which of the following, if true, best supports the contention in the last sentence?

Firstly, we have to understand the question stem that requires strengthen the last sentence means there are other existing reasons that causes negotiations successful
Now, let's look for the answer choice that provide this information

A. Predicting the success of a particular negotiation requires specifying the goal of the negotiation.

irrelevant. We're discussing about the cause of successful negotiations, not how to predict the success of a particular negotiation

B. Judging the outcome of a particular negotiation requires knowing about other negotiations that have taken place in the past.

irrelevant. We're discussing about the cause of successful negotiations, not how to judge the outcome of a particular negotiation

C. Learning whether a certain negotiation strategy is good requires observing how that strategy works through several negotiating sessions.

irrelevant. We're discussing about the cause of successful negotiations, not good or bad negotiation

D. Parties who are willing to take a symbolic step are more likely to complete negotiations successfully for other reasons.

Be careful with "[color=#0000ff]complete negotiations successfully for other reasons.". Although this choice does not indicate specifically the reason negotiations are resolved successfully but this sentence provides enough proof that there are other existing reasons that cause negotiations successful. Let's keep (D)[/color]

E. Making a negotiation successful requires knowing the context of symbolic steps that a party in the negotiations might desire.