The magnetic field strength along the axis (x'ox) of a solenoid is measured ( o being the center of the solenoid). The adjacent graph represent the length:

Question

(The graph represents the variation of B (mT) as a function of x (cm). The numbers on the y axis are 1 2 3 and on the x axis 10 20. The curve remains constant at 3mT and decreases when it reaches 20cm.)
A) What measuring device was used?
B) why is the value of the current located on the graph? (I=2A)
C) Deduce from the graph an approximate value of the length of the solenoid.
D) what is the number of turns per meter of the solenoid.

THANKS!