Suppose nominal GDP increased in a given year. Based on this information, we know with certainty that （ ）
选项：

A:real output has increased.
B:the price level (GDP deflator) has increased.
C:real output and the price level (GDP deflator) have both increased.
D:either real output or the price level (GDP deflator) have increased.
E:real output has increased and the price level has decreased.