How should you signal the crane operator to stop in an emergency?（）.

A.Extend arm with the palm down and and hold this position rigidly

B.Place clasped hands in frontof your body

C.extend arm and move hand rapidly right and left with the plam outward

D.Place both fists in front of the body with the thumbs pointing outward

配制c（1∕2H₂SO₄）＝0.0100mol∕L硫酸标准溶液，可由c（1∕2H₂SO₄）＝0.1000mol∕L硫酸标准溶液准确地稀释至（）倍制得。