"奧運會每四年舉行一次"英語怎麼說?

題目：

"奧運會每四年舉行一次"英語怎麼說?
1.奧運會每四年舉行一次
2.通常在春天種樹
3.我認爲需要建兩座橋.
----翻譯

解答：

1.Olympic games is held every four years .
2.Usually, trees are planted in spring .
3.I think two bridges need to be built.
I think two bridges need be built.（這句是把need看做是情態動詞,但是好象還是沒有上一句好）