A line graph displays sales growth for two investments over 4 years:

• Investment X: Year 1 = $500, Year 2 =$900, Year 3 = $1,300, Year 4 =$1,700
• Investment Y: Year 1 = $1,000, Year 2 =$1,400, Year 3 = $1,600, Year 4 =$1,700

During which period is Investment X's rate of growth (per year) greater than Investment Y's?