A ball is dropped from rest from the top of a tower of height $H$. At the exact same instant, a second ball is projected vertically upwards from the ground at speed $u$. The two balls pass each other at a height of $\frac{3}{4}H$ above the ground. In terms of $H$ and acceleration due to gravity $g$, what is the initial projection speed $u$?

• A$\frac{1}{2}\sqrt{gH}$
• B$\sqrt{gH}$
• C$\sqrt{2gH}$
• D$2\sqrt{gH}$

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