I could not buy from apps,I forgot the security question

Call Apple to help reset your Security Question.

http://support.apple.com/kb/HT5699

See Kappy's great User Tips.

See my User Tip for some help: Some Solutions for Resetting Forgotten Security Questions: Apple Support Communities https://discussions.apple.com/docs/DOC-4551

Rescue email address and how to reset Apple ID security questions

http://support.apple.com/kb/HT5312

Send Apple an email request for help at: Apple - Support - iTunes Store - Contact Us http://www.apple.com/emea/support/itunes/contact.html

Call Apple Support in your country: Customer Service: Contacting Apple for support and service http://support.apple.com/kb/HE57

 Cheers, Tom 😉