Grouped Self-Constraint
Strong and weak links alternate along a chain, and its ends force a conclusion.
All candidates have been entered in the empty cells.
A marks the same candidate at both ends of the chain. Assume A is false. On a solid link, one false end makes the other end true. On a dashed link, one true end makes the other end false. Applying these rules in link order makes A true, contradicting the assumption. Therefore, cell B is 5.