设有直线m，n和平面α、β，下列四个命题中，正确的序号是______．

（1）若m∥α，n∥α，则m∥n

（2）若m⊂α，n⊂α，m∥β，n∥β，则α∥β

（3）若α⊥β，m⊂α，则m⊥β

（4）若若α⊥β，m⊥β，m⊈α，则m∥α

A new technique_________, the yields as a whole increased by 20 percent.

A．working out

B．having worked out

C．having been worked out

D．to have been worked out