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1. A box contains nine bulbs out of which 4 are defective. If four bulbs are chosen at random, find the probability that all the four bulbs are defective.

A. $\frac{62}{63}$

B. $\frac{125}{126}$

C. $\frac{1}{63}$

D. $\frac{1}{126}$

Solution:

Out of nine, five are good and four are defective.
Required probability

\begin{aligned} = \frac{^{4} C_{4}}{^{9} C_{4}} \\ = \frac{1}{126} \end{aligned}

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