17. Compare the absolute values of the measurement results of these six parts:

0.2 0.3 0.2 0.3 0.4 0. 1

Obviously, the part with an absolute value of 0. 1 is closest to the specified size, so the sixth part is the best.

Next question:

Because a+b=0, cd= 1, l m l=2.

Therefore [(a+b)/(a+b+c)]+m-cd.

=[0/(0+c)]+m- 1

=m- 1

= 1 or -3

18, obviously b>0> and l b l > l a l

So l a l -l b l=-a-b

19、a & lt-b & lt； 0 & ltb & lt-a & lt； letter of credit (L/C)

20、l -a l=2

Then a= 2 or -2, and the answer is C.