An object is moving to the west at a constant speed. Three forces are exerted on the object. One force is 5 N directed due north, and another is 5 N directed due west. What is the magnitude and direction of the third force if the object is to continue moving to the west at a constant speed? 
选项为:A:$5\sqrt{3}$~N, directed northwest  B:$5\sqrt{3}$~N, directed southeast  C:$5\sqrt{2}$~N, directed northwest  D:$5\sqrt{2}$~N, directed southeast  请回答其中一个选项.