Find what the supply sees through a bridge

A bridge has upper/lower resistors 2 Ω and 4 Ω on the left, and 4 Ω and 2 Ω on the right. A 4 Ω cross-link joins the middle nodes. Apply an ideal 10 V source between the top and bottom rails.

What is the equivalent resistance seen by the source?

Organize the givens & assumptions

Left arm: 2 Ω then 4 Ω
Right arm: 4 Ω then 2 Ω
Middle cross-link: 4 Ω

Settled lumped DC circuit, ideal connecting wires, positive constant Ohmic resistances and no changing external magnetic flux. Sources are ideal except for explicitly specified series resistances.

A $4\ \Omega$
B $12\ \Omega$
C $20/7\ \Omega$
D $3\ \Omega$

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