Why is fluoride varnish beneficial?

FLUORIDE VARNISH advantages: (1) HIGH BIOAVAILABILITY — sustained fluoride release at high concentrations at the tooth surface; (2) SUBSTANTIVITY — prolonged retention/contact time with tooth structure compared to rinses; (3) Favorable pH — acidic pH promotes fluoride incorporation into enamel as fluorapatite; (4) Easy professional application. The combination of high bioavailability AND substantivity makes varnish superior to other topical fluoride forms for caries prevention. A is correct.