Question

The electrical circuit shown contains four light bulbs, four switches and four dry cells. What is the minimum number of switches that has to be closed in order for all the light bulbs to be lit ?

• A. $1$
• B. $2$
• C. $3$
• D. $4$

Answer 1

The correct option is C $3$

Naming the switches from top to bottom (A,B,C,D)

Naming light bulbs from left to right (1,2,3,4)

A & B need to be closed, we can see directly that otherwise (1,2) would lie in an open circuit.

Between switches C and D, either of them can be open (anyone will do) and both the bulbs 3,4 would glow.

Hence the minimum number of switches needed is 3.