Two blocks, which can be modeled as point masses, are connected by a massless string which passes through a hole in a frictionless table. A tube extends out of the hole in the table so that the portion of the string between the hole and $M_1$ remains parallel to the top of the table. The blocks have mass $M_1=3kg$ and unknown mass $M_2.$ Block $1$ is at a distance $r=2\mathrm{.}5m$ from the center of the frictionless surface and is rotating around the table with a period $T=$ s$.$ Block $2$ hangs vertically underneath and does not move relative to the table. What is the mass of $M_2?$ Use $g=10\frac{m}{s^2}.$

A$.1$ kg

B$.2$ kg

C$.2\mathrm{.}5$ kg

D$.3$ kg

E$.10$ kg