In the force method canonical equations main coefficients δii _____ zero; secondary coefficients δij (i ≠ j) _____ zero.
选项：

A:are greater than; may be greater than, smaller than or equal to;
B:are smaller than; may be greater than, smaller than or equal to;
C:are equal to; are greater than;
D:may be greater than, smaller than or equal to; are equal to;
E:are greater than; are greater than;
F:may be greater than, smaller than or equal to; are greater than