scoring and structure bach scored the work in d major for four vocal soloists ( soprano ( s ) , alto ( a ) , tenor ( t ) and bass ( b ) ) , a satb four-part choir , and a baroque orchestra of two traverse flutes ( ft ) , two oboes ( ob ) , two horns ( co ) , bassoon ( fg ) , two violins ( vl ) , viola ( va ) and basso continuo ( bc ) , with three trumpets and timpani ad libitum .