You have four test tubes A, B, C and D filled with aqueous solution of same concentration but of different salts. In A solution of NaCl, B solution of CaCl2, C solution of MgCl2 and D solution of KCl is filled. If you add equal amounts of a soap solution to each test tube and shake the contents well, in which of the test tubes will scum (insoluble substance) be obtained ?
(a) A and B (b) B and C
(c) C and D (d) A and D
