B, in order to reduce the error, the tension should be as large as possible, so B is correct.

C. When pulling the rubber band, the rubber band, string and spring scale should be close to the drawing board and parallel to the drawing board. So, C is correct.

D. Before the experiment, hook the hooks of the two spring scales, lay them flat on the table, and pull them in the opposite direction to check whether the readings are the same. Therefore, d is correct.

(2)① In the experiment of verifying the law of conservation of mechanical energy, it is necessary to verify whether the increase of kinetic energy and the decrease of gravitational potential energy are equal, and the mass is measurable but unpredictable, so it is not necessary to measure the weight of the heavy hammer with a balance. Tick-tock timer should be connected to AC power supply, not DC power supply. Therefore, the unnecessary step is a, and the inappropriate step is C.

② According to the differential method, a = (S4+S5+S6)? (s 1+s2+s3)9T2, and substitute the data to get a = 9.58m/S2.

③ Unreasonable. Because of the existence of resistance, the result obtained in problem (2) is not local gravitational acceleration, so it cannot be used to calculate the change of gravitational potential energy.

So the answer is: (1)BCD.

(2)( 1)A，C (2)9.58

(3) Unreasonable. Because of the existence of resistance, the result obtained in problem (2) is not local gravitational acceleration, so it cannot be used to calculate the change of gravitational potential energy.