Choose the best topic sentence from the four options below.

The first electronic computer, ENIAC, went into operation just over fifty years ago. Compared with today’s home computer, ENIAC was enormous, consisting of forty panels, each two feet wide and four feet deep. Today’s PC or Macintosh, by contrast, can fit on one’s desk or even lap. ENIAC had to be configured by hand, with its programmers taking up to two days to reset switches and cables. Today, the average home user can change programs in an instant. And for all its size and inconvenience, ENIAC was slow. In its time, its operating speed of 100,000 pulses per second seemed amazingly fast. However, today’s home machine can operate at 1 billion pulses per second or faster.