答案是：
✘ The reason is because the system is too expensive.
✓ The reason is that the system is too expensive.
這句的意思是「因為那系統太貴」。The reason is... 意思是「原因是...」已經有「因為」這個意思在裡面，所以不用再加because進去，否則會意思重疊。
但是由於後面的這個原因是一句完整句子「the system is too expensive.」所以我們需要加 that。如果只是一個名詞就不用，例如：The reason is the bad weather.
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