GENEVA: Rebeca Grynspan is upbeat about her chances of becoming the next head of the United Nations, which she insists must become more agile in tackling the world’s crises. The Costa Rican former vice president said she wanted to rebuild global trust in the United Nations if she becomes its next secretary-general. “We are very optimistic. I think that I am more than a viable candidate,” Grynspan said on Friday, her last working day before stepping aside as head of the UN trade and development agency UNCTAD to focus on her campaign.