Which is wrong about nucleic acids? 
(a) DNA is single stranded in some viruses
(b) RNA is double stranded occasionally
(c) Length of one helix is 45 Å in B-DNA
(d) One turn of Z-DNA has 12 bases
(c) Length of one helix or the pitch per turn of helix is 34 Å in B-DNA, 25 Å in A-DNA and 46 Å in Z-model of DNA. In Z-DNA sugar molecules are seen in opposite direction. So the 3-5 diester bond forms zig-zag structure in Z-DNA.