A、One. B、Two. C、Three. D、Four. D原文提到 "in early August they made their fourth trip"，注意这里使用的是过去时，也就是说已经完成了第四次旅行，因此选D。

It was a glorious day when Steve Dobson realized the mountain climbing gene planted deep in his soul had been passed on to his daughter, Liza. The father and daughter have become quite a team, and in early August they made their fourth trip to Colorado for what has become an annual tradition for them.
    When Steve was a young man and a student at Bluffton College, he had the opportunity to take a mountain climbing class for credit. The class gave him the chance to not only learn about mountains, but also his first trip west to actually be in the mountains. It was love at first sight. "I loved being in the mountains," he said. "Being on top of the mountain was an unbelievable experience."
    But life took him in another direction. Fast forward a few years and Steve found himself living in Maysville with a family and the owner of D. H. Resorts in Fleming County. Steve, his wife, and two children frequently headed west to Colorado for family vacations to go snow skiing.
    Four years ago they made their first trip to Colorado as a family in the summer. The active family spent part of that vacation hiking on trails, enjoying summer in the mountains. It gave Steve an idea. "I got the idea that we would climb a mountain as a family."
    He chose a mountain that was just over 12,000 feet, and since he hadn’t tried it since he was in college, he wasn’t sure how difficult it would be. "Mountain climbing is not easy," he said. "It’s not a hike in the woods. But it’s also not as technical as rock climbing."