Ac Alternating current (ac) is electric current that alternates between a positive maximum value and a negativemaximum value at a characteristic frequency, usually 50 or 60 cycles per second (Hertz).
交流交流是指电流在正负最大值之间以特定的频率变动，通常频率为50或60赫兹。
2
To cast it in less negative terms, it is better to think of this as taking advantage of a rare opportunity — and it is too good a one not to lever to its maximum potential.
但是从积极的角度来考虑，这不啻为一次难得的机会——不利用来发挥自己的最大潜力实属可惜。
3
A great portrait photographer considers shapes, lines, framing, angles, negative space, where to place the point of focus in the frame for maximum impact, and so on.