Answer:
Both options C and D are correct.
Explanation:
Inflation refers to an increase in the general price level of goods and services overtime. Since it is conveyed in the question that the general price level in a later year became twice as high, inflation definitely occurred. Hence, option D is correct.
Nominal GDP is the value of the total output at current market prices. Real GDP adjusts that value for inflation. As prices double, nominal GDP ought to increase from $400m to $800m. However, it actually rose to $1000m. This additional increase of $200m shows that the real GDP has risen. However, the increase in real GDP is less than 100%. This implies option C is also correct.