There are two pumps to fill a tank with water. First pump can fill the empty tank in 8 hours, while the second in 10 hours, If both the pumps are opened at the same time and kept open for 4 hours, the part of the tank that will be filled up is :
[A] $\frac{2}{5}$
[B] $\frac{1}{5}$
[C] $\frac{9}{10}$
[D] $\frac{1}{10}$