# When a sample of human blood is diluted $200$ times its initial volume and microscopically examined in a layer $0.10\;mm$ thick , an average of $30\;RBC$ are found in $100 \times 100$ micrometer square. The number of RBC in $1 \;mm^3$ of undiluted blood is
$(a)\; 3 \times 10^6 \\ (b)\;6 \times 10^6\\(c)\;10^6\\(d)\;2 \times 10^6$