Time proportioning control: A. can be used on a PM550 programmable process controller B. is less efficient than normal PID control C. cannot use a TM990 micro¬computer D. a and b above. E. None of the above

Option B is incorrect because time proportioning control is not necessarily less efficient than normal PID control. In some cases, it may be more efficient.

Option C is incorrect because time proportioning control can use a TM990 micro¬computer.

Option D is incorrect because only option A is correct.

Option E is incorrect because option A is correct.