\textbf{154-} In a box there are $8$ light bulbs, two of which are defective. The bulbs are tested randomly one by one and the good bulb is set aside, until the first defective bulb is found. In the third test, what is the probability of finding the first defective bulb?
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(1) $\dfrac{5}{28}$ \hspace{2cm} (2) $\dfrac{4}{21}$ \hspace{2cm} (3) $\dfrac{3}{14}$ \hspace{2cm} (4) $\dfrac{5}{21}$
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