6. It is known that a box contains 3 screw-base and 7 bayonet-base bulbs, which have the same shape and power and are placed with their bases down. Now, a bayonet-base bulb is needed for use, and the electrician takes one at random each time without replacement. The probability that he gets a bayonet-base bulb only on the 3rd try is
A. $\frac{21}{40}$
B. $\frac{17}{40}$
C. $\frac{3}{10}$
D. $\frac{7}{120}$
6. $\mathrm{D} \quad P=\frac{3}{10} \times \frac{2}{9} \times \frac{7}{8}=\frac{7}{120}$