单选题 ______ have been made for nearly three decades to increase the amount of precipitation from clouds by seeding them with salt or silver iodide.

【正确答案】 C

【答案解析】[分析]名词辨析 A．Devices装置，设计，策略，设备；B．Hypotheses臆测，假定；C．Attempts努力，尝试，企图；D．Commissions委任，代办(权)，佣金；本题讲的是：已做了近三十年的尝试，试图在云层中加入盐或碘化银来增加降雨量；因此C为答案。