这个是HOTS question， 只要是合理的答案都可被接受
我的答案： David's father promised that he would take them to the beach again because they didn't manage to have a picnic that day due to unexpected rain. Besides, his father wants to let them experience picnics and let them close to nature.
翻译：大卫的父亲答应他会再带他们去海滩，因为那天他们因意想不到的大雨没能去野餐。 此外，他的父亲想让他们体验野餐，让他们亲近大自然。
