(1) Because the glass plate used by students in group A is not coated, the glass plate used by students in group B has a thin coating on the back, which can reflect light and transmit light. Therefore, the images of students in group B are clearer.

(2) When the images of object B and object A are completely coincident, it is convenient to compare the size relationship of object images. Therefore, taking a candle that is the same size as a lit candle and completely coincides with the image shows that the image is exactly the same as an unlit candle, and an unlit candle is the same as a lit candle. This experimental method is called equivalent substitution method.

When measuring object distance and image distance, measure the distance from the candle to 2.

(3) According to the data in the table, we can see that: Error: the next digit of the dividing value was not estimated when recording the data;

Disadvantages: When summing up the experimental conclusions through experiments, it is necessary to carry out many experiments before the experimental conclusions are universal. There is only one experiment, and the number of experiments is too small, so the conclusion is very accidental.

So, the answer is: (1) b; Law of reflection of light;

(2) A; ? 2;

(3) Error: When recording data, the next bit of the division value was not estimated;

Insufficient: It is accidental or not universal to draw conclusions from only one set of data.