当 t（     ）时，关于x的方程x²-3x+t=0可用公式法求解。

How should you signal the crane operator to swing?（）.

A.With both arms extended out and palms to down, move arms back and forth

B.With arm ertended downwards and forefinger pointing down,move hand in small horizontal crecles

C.Extend arm and point finger in the direction to move boom

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