(20 1? Nanning Sanmao) Xiaoming's family once saw the conductor "smoking" when using the microwave oven, which almost caused a fire. As shown in figure 1, it is the scene at that time, home.

(1) Because in the parallel circuit, the main current is equal to the sum of the current of each branch, when all the appliances in 4 1 are working, the current in the main circuit will be very large, which will lead to the phenomenon of wire breakage, so the connection mode of each appliance in the household circuit is parallel;

(2) Usage of the test pen: When using the test pen, the hand should touch the metal body at the tail of the test pen, and the nib should touch the wire. Neon tube glows to indicate contact with the fire line, and if it doesn't glow, it is the zero line. Among 42 A and B, 4 B is the right choice to use electric probe.

So, the answer is: and; B.