Figure 3.7 illustrates two single-phase transmission lines. The Geometrical Mean Radius (GMR) of each conductor is (r^{prime}).

Question:

Figure 3.7 illustrates two single-phase transmission lines. The Geometrical Mean Radius (GMR) of each conductor is $r^{\prime}$. In Fig. 3.7 (b), conductors " 2 " and " 3 " are for sending power, and conductor " 1 " is for receiving power. What relation should be held between $D$ and $r^{\prime}$ so that the inductances of the transmission lines become equal?

1) $D=\frac{9}{4} r^{\prime}$

2) $D=\frac{37}{16} r^{\prime}$

3) $D=\frac{5}{2} r^{\prime}$

4) $D=\frac{21}{8} r^{\prime}$

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