A company's quarterly sales figures for a new product are as follows: Year 1: units, Year 2: units, Year 3: units, Year 4: units. Which of the following statements best describes the trend shown in the line graph representing these sales figures? 
Sales increased at a constant rate throughout the year.
Sales decreased after an initial increase.
Sales increased, but the rate of increase slowed down over time.
Sales increased at an accelerating rate.
Sales fluctuated significantly, with no clear overall trend.
