① Comparing the experimental data of 1, 2, 3 or 4, 5 and 6, it can be seen that the contact area and roughness are the same, and the mass of the object is different, so the indicator of dynamometer increases gradually with different pressures, from which a preliminary conclusion can be drawn: if the object is pulled at a uniform speed along the horizontal direction on the same horizontal plane, the greater the pressure, the greater the pulling force;

② By comparing the data of experimental serial number 1 and 4 or 2 and 5 or 3 and 6, we can know that the roughness of contact surface is different when the pressure is the same and the object moves on different surfaces; From this, a preliminary conclusion can be drawn: on the horizontal plane, the rougher the contact surface, the greater the tension;

③ The analysis of the experimental conclusions in Table 3 shows that the contact area, roughness and pressure are the same, but when the angle between tension and the horizontal plane increases, the tension first decreases and then increases. From this analysis, it can be concluded that when the contact area, roughness and pressure are the same, the object is pulled at a uniform speed along the horizontal direction, and when the angle between the tension and the horizontal plane increases, the tension first decreases and then increases;

According to the above conclusions, it can be judged that when the angle between the pulling force and the horizontal plane is at least 30 degrees, the pulling force is the smallest; Therefore, in order to find this angle, we should further explore it within the range of included angle α of 20 ~ 40 degrees.

④ According to the control variable method, the contact area and pressure should remain unchanged. When the observation angle α is 30 degrees, the pulling force moves at a uniform speed on horizontal planes with different roughness. Is the pulling force the smallest? The experimental steps are as follows:

1. Put blocks with the same mass on horizontal planes with different roughness.

2. Pull the animal block evenly in different directions.

3. Record data.

So the correct steps: b, e, f.

So the answer is: ① The greater the pressure, the greater the tension;

(2) On the horizontal plane, the object is pulled at a uniform speed along the horizontal direction, and the rougher the contact surface, the greater the pulling force;

(3) When the contact area, roughness and pressure are the same, the object is pulled at a uniform speed along the horizontal direction. When the angle between the tension and the horizontal plane increases, the tension first decreases and then increases; 20~40;

④B、E、f。