Faced with a threat of entry, the monopolist can invest in research and development in ( ) and earn a smaller profit in ( ); however, if the monopolist’s lower costs in period 2 deter entry, the monopolist’s profits will be significantly increased in( )
选项：

A:period 1, period 2, period 1.
B:period 2, period 1, period 1.
C:period 2, period 1, period 2.
D:period 1, period 2, period 2.