单选题 Uncomfortable with their mean jokes, he tried to raise the tone of the conversation.
【正确答案】 B
【答案解析】[解析] 句意：对他们下流的玩笑感到不适，他试图提高对话的水准。raise提升；养育。elevate提升，提高。alleviate减轻，缓和。gravitate受重力作用；被吸引。leaven使发酵。