(2) According to the meaning of the question, the resistance of apple is related to sweetness. Now it is necessary to study the relationship between the resistance of apples and the shape of apples to keep the sweetness of apples unchanged.
Scheme A: This scheme can better control the influence of apple sweetness on apple resistance and is easy to operate.
Scheme B: Theoretically feasible, but not feasible. It is difficult to get apples with the same sweetness but different shapes.
At the same voltage, the measured currents are all the same, indicating the same resistance, indicating that the resistance of the apple has nothing to do with the shape of the apple.
(3)① Draw corresponding points on the coordinate paper according to the experimental data, and then connect these points with smooth curves to get the image as shown in the figure.
② According to the experimental data, when the voltage is constant, the greater the sugar concentration, that is, the sweeter the apple, the greater the current, and according to Ohm's law, the smaller the resistance of the apple.
③ According to the data, the sugar concentration increased by 8mg? Ml- 1, the current increased by 40 μ a; Sugar concentration increased 14mg? Ml- 1, the current increased by 70 μ a; Sugar concentration increased 12mg? Ml- 1, current increased by 60μA, etc. Numerically, the current increases the sugar concentration by five times. Then 5(P-32)=I- 10, I = 5p-150;
The resistance is constant, and the current is proportional to the voltage: I300μA=24V30V, and I=240μA is calculated and substituted into I=5P- 150.
240μA=5P- 150，P=78mg/ml。 Because 78mg/ml is less than 80mg/ml, apples are not sweet.
So the answer is: (1) The circuit diagram is as shown in figure 1; (2) A; Scheme A can better control the sweetness of apples; The resistance of apple has nothing to do with the shape of apple;
(3)① The image is shown in Figure 2; ② small; ③I = 5P- 150； Not sweet.