Draw the Feynman diagrams for the gluon exchanges for the following couples of vertices: 

(a) ^Rq → ^Bq, ^Bq → ^Rq; 

(b) ^Rq→^Bq, ^R̅q→^B̅q; 

(c) ^Rq→^Rq, ^R̅q→^R̅q. Specify which gluon (s) is (are) exchanged and the colour charges at both vertices. 

(d) Explain the meanings of the signs of the results.