题目

Mr. Fang is now the owner of 36 greenhouses producing organic vegetables in the Gobi desert in Northwest China’s Gansu province. He is one of the beneficiaries of the Gobi Farming Program of Gansu province that is building rows of greenhouses in the desert to help transform local farming and alleviate poverty.Grain production in Gansu is greatly affected by the climate and the output is unstable. Agricultural production there relies heavily on rivers, oases and groundwater irrigation. If traditional extensive agricultural production continued, it could gradually narrow the living space.In 19在等比数列{an}（n∈N*）中，若a1=1，a4=18，则该数列的前5项和为（　　）