单选题：Which of the following statement is incorrect?
Which of the following statement is incorrect? @[B](1)
A. Monolithic OS is usually difficult to modify.
B. Micro-kernels allow some system services to be implemented just as user programs.
C. Layered OS is more efficient than monolithic OS.
D. Virtual machines improve OS development and testing process.
A.Monolithic OS is usually difficult to modify.
B.Micro-kernels allow some system services to be implemented just as user programs.
C.Layered OS is more efficient than monolithic OS.
D.Virtual machines improve OS development and testing process.
答案：B
A. Monolithic OS is usually difficult to modify.
B. Micro-kernels allow some system services to be implemented just as user programs.
C. Layered OS is more efficient than monolithic OS.
D. Virtual machines improve OS development and testing process.
A.Monolithic OS is usually difficult to modify.
B.Micro-kernels allow some system services to be implemented just as user programs.
C.Layered OS is more efficient than monolithic OS.
D.Virtual machines improve OS development and testing process.
答案：B