[单选题]
The grand unified theory (GUT) is a sob+ -aqa;g p:.brzuvdr 3 rl4kc,q4oh3ught-after model t5.xxug 8u lumo4k ii4;hat would unify three of the four forces of nature. Which fog44 .;lo imu8 uxki5xurce is the one it would not include?
A. gravitational
B. strong
C. weak
D. electromagnetic
