单项选择题
As mentioned above, C imposes relatively few built - in ways of doing things on the program- mer. Some common tasks, such as manipulating （1） , （2）, and doing （3） (I/O), are performed by calling on library functions. Other tasks which you might want to do, such as creating or listing directories, or interacting with a mouse, or displaying windows or other （4） elements, or doing color graphics, are not defined by the C language at all. You can do these things from a C program, of course, but you will be calling on services which are peculiar to your programming environment （5）, processor, and operating system) and which are not defined by the C standard. Since this course is about portable C programming, it will also be steering clear of facilities not provided in all C environments.
（1）处应选择（）
A.lines
B.strings
C.threads
D.routes