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}

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