CONCEPT: The question tests on the basic skills of Geometry

SOLUTION:

Sum of angles in a triangle =180 degree
Hence third angle opposite to BC(angle A) = 180-(70+20)=90 degree

In any triangle, the side opposite to larger angle is longer.
In ascending order 20<70<90 =>AC<AB<BC

Also sum of two sides is greater than the third side in a triangle.
 Hence (D)