A
A.$frac{{rm{d}}}{{{rm{d}}t}}left( {frac{{partial T}}{{partial {{dot q}_k}}}} right) - frac{{partial T}}{{partial {q_k}}} = 0$
B
B.$frac{{rm{d}}}{{{rm{d}}t}}left( {frac{{partial T}}{{partial {{dot q}_k}}}} right) - frac{{partial T}}{{partial {q_k}}} = {Q_k}$
C
C.$frac{{rm{d}}}{{{rm{d}}t}}left( {frac{{partial T}}{{partial {{dot q}_k}}}} right) + frac{{partial T}}{{partial {q_k}}} = {Q_k}$
