根据短文内容选择正确的答案。
Are you learning English and do you need help with grammar? Do you want to learn new vocabulary or slang (俚语)? Well, you can find help online with these top 5 free English as a Second Language websites.
Dave's ESL Cafe
This is a complete website that can help you learn and practise English. You can read many stories and do a lot of exercises to test yourself. You can also post questions and answers onto the help centre.
About.com
You can get free English courses from About.com and you’ll get regular e-mails that will help you learn English.
The Classroom by Voice of America
You can watch videos and listen to news stories to improve your listening and vocabulary skills. You can also hear English words and their meanings.
Heinle's Newbury House Dictionary of American English
Do you need to find the definition of a word? Use this online dictionary to find simple and clear definitions, as well as example sentences and idioms (习语). You can also find different cultural facts.
Personal English Portal (门户网站) on YouTube
You can watch this series of videos on YouTube to develop your listening skills. You can also do a search for “learn English” on YouTube to find other videos. This is a good way to practise English and have fun.
，则向量a与b的夹角为 （ ）