Question

【题目】People volunteer to express personal values, to expand their range of experiences, and(strength) social relationships.

Answer 1

【答案】to strengthen
【解析】句意：人们自愿表达个人价值观，扩展经历范围并且增强社会关系。根据前面的to express personal values, to expand，可以判断出三个不定式动词并列。故填to strengthen。