bing,yahoo,google三大搜尋引擎的safesearch相關連結

1. google：如何在校內封鎖成人內容：https://support.google.com/websearch/answer/186669?hl=zh-Hant
2. Bing：Block adult content with SafeSearch
3. Yahoo：About Yahoo SafeSearch
4. Yahoo：Yahoo Search Safety Guide