______ it to clear up tomorrow, the tourists would climb to the top of the mountain to wait for the sun to rise.

【正确答案】 B

【答案解析】 根据“would climb”可知，考查与将来事实相反的虚拟语气。由条件句中的“to”可知，条件句谓语形式为：were to，可以省略if，并把were提前到句首。因此选B。