#### Answer

$ 2\times10^{-9}\ m$

#### Work Step by Step

Volume of oil in $m^3$: $1\ t.s\times\frac{5\ cm^3}{1\ t.s.}\times\left(\frac{1\ m}{100\ cm}\right)^3=5\times10^{-6}\ m^3$
Area of water in m²: $0.5\ acre\times \frac{1\times10^4\ m^2}{2.47\ acres}=2\times10^3\ m^2$
The volume being given by: $V=A.t$
$t=5\times10^{-6}\div2\times10^3 = 2\times10^{-9}\ m$
Given that this is in the range of nm, this is propably the molecular size.