同学你好，该知识点来自沪江网校《大学水平直达雅思7分【剑桥全能随到随学班】》的课程，想要更系统的学习，欢迎进入课程学习。不仅可以和更多的同学一起学习，而且还有老师、助教随时的学习指导和知识点解答哦。

同学你好，
The main reason is that many people did not realize how serious environmental problems could be.
这句话里that 接表语从句（is后面是表语成分），不能与because同时接，即The main reason is that because many people did not realize how serious environmental problems could be.是不对的。主要原因是许多人没有意识到环境问题有多严重，many people xxx本身就是原因，不用再加because哦

希望对你有帮助o(∩_∩)o