苹果笔记本只有电源键能用的解决办法

因修改usb驱动一部分文件，致mac book pro重启后，键盘，usb等外设全部失效，然后不停的google，重要找到方法了

在按启动的时候：按住command + R 就可恢复系统，

本以为会导致数据丢失，但是发现有重装系统的的选项，这样就安心多了。

http://support.apple.com/kb/ht1533

Keystroke	Description
Press C during startup	Start up from a bootable CD, DVD, or USB thumb drive (such as OS X install media).
Press D during startup	Start up in Apple Hardware Test (AHT).
Press Option-Command-P-R until you hear startup sound a second time.	Reset NVRAM
Press Option during startup	Start up in Startup Manager, where you can select an OS X volume or network volume to start from.
Press Eject, F12, or hold the mouse or trackpad button	Ejects any removable media, such as an optical disc.
Press N during startup	Attempt to start up from a compatible network server (NetBoot).
Press T during startup	Start up in Target Disk Mode.
Press Shift during startup	Start up in Safe Boot mode and temporarily disable login items.
Press Command-V during startup	Start up in Verbose mode.
Press Command-S during startup	Start up in Single-User mode.
Press Option-N during startup	Start from a NetBoot server using the default boot image.
Press Command-R during startup	Start from the OS X Recovery System1