OG13: Along the major rivers that travers the deserts of northeast Africa, the Middle East, and northwest India, the combination of a reliable supply of water and good growing conditions encoraged farming traditions that have, in places, endured for at least 6000 years
假设是现在时,the combination of + a reliabble ~ and good growing ~后面的谓语动词用单数还是复数?combination还有没有其他情况?
求大神指教