Which of the following sentences can be used to indicate a research gap in the Introduction Section?
选项：

A:It is generally accepted that joints in steel frame operate in a semi-rigid fashion.;
B:For more than 100 years, researchers have been observing the stress strain behavior. ;
C:The potential for development, therefore, seems to be very promising. ;
D:However, light scattering techniques have been largely unsuccessful to date.