【正确答案】 1、the oil shortages．    

【答案解析】[解析] 本题涉及人们重新关注天然气燃料的原因。小标题From Field to Ford下第二段最后一句The oil shortages of the late 1960s and early 1970s brought renewed interest in natural gas as a fuel source，especially for automobiles为答案出处。题干对原文的句式进行了转换，resumed their interest对应文中的brought renewed interest。