A Zener diode with a breakdown voltage of $6\ \text{V}$ can safely handle at most $1.2\ \text{W}$. It is connected in reverse bias, in series with a resistor $R$, across a $10\ \text{V}$ supply, as shown in the figure, with no load connected. The smallest value of $R$ that keeps the diode safe is

Fig. 1: A 10 V supply in series with a resistor R and a reverse-biased Zener diode rated 6 V, 1.2 W.