The two sentences describe a contrasting relationship. The first sentence highlights the positive action of Bill Gates, who has donated a large sum of money to fight against polio. This presents a positive view of his efforts. However, the second sentence introduces a different perspective by mentioning that some critics have spoken out against Gates's efforts. This indicates a contrasting viewpoint and creates a sense of disagreement or criticism towards his actions. In summary, the relationship between the two sentences is one of contrast, where the first sentence presents a positive view and the second sentence introduces criticism.