"Complementarity" refers to the fact that
in Watson-Crick base-pairs, an A on one strand of the DNA double helix always pairs with a T on the other strand, and similarly a G on one strand always
pairs with a C on the other strand
in Watson-Crick base-pairs, an A will always pair with another A, a T with another T, and so on
in Watson-Crick base-pairs, all the A's are on one strand of the DNA double helix, and all
the T's are on the other
in Watson-Crick base-pairs, an A always pairs with the T next to it on the same strand of the DNA double helix, while on the other strand, a G always pairs
with the C next to it on that strand
