那是

• that's
• which

例句

1. 那是一段艰难的日子。
   That was a hard time.
2. 我喜欢回顾我的中学时代，那是我生命中最快乐的时光。
   I like to look back on my high-school days, which were the happiest in my life.
3. 电视台为出现的干扰表示歉意，那是由于恶劣的天气状况造成的。
   The television station apologized for the interference, which was due to bad weather conditions.
4. 那是一个以几种语言叙述的古老的故事。
   It's an ancient tale which appears in various guises in several languages.
5. 那是我的两个儿子。你能猜出哪个大吗？
   There go my two sons. Can you guess which is the elder?