If the price is reduced by 5%, the price will be 100×( 1-5%)=95 yuan.

Then the original profit of each pair is 100-x yuan, and the profit is 80( 100-x)=8000-80x.

Now the profit per pair is 95-x yuan.

Price reduction 100-95=5 yuan.

So the sales volume increased by 4×5=20 pairs.

So it's 80+20= 100 pairs.

Profit100 (95-x) = 9500-100x.

So the profit increased (9500-100x)-(8000-80x) =100.

1500-20x= 100

1400=20x

x=70

A: The charge is 70 yuan.