The correct answer is: A. Can be obtained by graphic statics.
Graphic statics is a method of solving statically determinate trusses by using a graphical representation of the forces acting on the truss. The method was developed by the Swiss engineer Karl Culmann in the 19th century.
To use graphic statics, the forces acting on the truss are represented by vectors. The vectors are then combined using the rules of vector addition and subtraction. The resulting vectors represent the forces in the members of the truss.
Graphic statics is a powerful tool for solving statically determinate trusses. It is relatively easy to learn and use, and it can be used to solve problems that would be difficult or impossible to solve using other methods.
Option B is incorrect because graphic statics can be used to solve statically determinate trusses. Option C is incorrect because graphic statics is a reliable method for solving statically determinate trusses. Option D is incorrect because graphic statics does not require trial and error.