The Acme Stereo Company had a capital stock of $24 million at the beginning of the year. At the end of the year, the firm had a capital stock of $20 million. Thus its
选项：

A:net investment was some amount but we need more information to determine the amount.
B:net investment was $4 million for the year.
C:gross investment was zero.
D:net investment was -$4 million for the year.