Answer: D. MTBF=70000h.
Analysis:
When discussing the reliability of CNC systems or CNC machine tools, MTBF (Mean Time Between Failures) is an important indicator. The longer the MTBF, the longer the average operating time of the equipment between two failures, that is, the higher the reliability of the equipment.
Now, let's analyze the four options given in the question:
A. MTBF=1000h: This is the shortest MTBF among the four options, indicating that the average operating time of the equipment between two failures is the shortest, so its reliability is relatively low.
B. MTBF=2500h: Although it is longer than the MTBF of option A, it is still in the shorter range among the four options, so its reliability is not the highest.
C. MTBF=5000h: This MTBF is longer than the first two options, but it is still not the longest, so although its reliability has improved, it is not the best.
D. MTBF=70000h: This is the longest MTBF of the four options, indicating that the average operating time between two failures of the equipment is the longest, so its reliability is the highest.
In summary, according to the MTBF index, the most reliable CNC system or CNC machine tool is the one with the longest MTBF, that is, option D: MTBF=70000h.
Therefore, the answer is: D.