Twenty (20) items were subjected to accelerated test conditions of 150°C for 400 hours. Two (2) items failed, one at 250 hours and one at 300 hours. If the acceleration factor was 2.0 and the distribution was exponential, the estimated MTTF under normal use conditions lies within which of the following ranges: MTTF 5,000 hours, 2,000 MTTF 5,000 hours, C, 1,000 MTTF 2,000 hours, MTTF 1,000 hours.