Drinking water is allowed a lead content of 50 ppb (that is, $50\mu \mathrm{g}$ per litre). The mass of lead in $2.0\mathrm{L}$ of water at exactly this limit is:

1. A$1.0\mathrm{mg}$
2. B$0.10\mathrm{g}$
3. C$10\mu \mathrm{g}$
4. D$100\mu \mathrm{g}$