3,5-dimethylpyrazole; urea-based blocking agents such as urea, thiourea, ethyleneurea, ethylenethiourea and diphenylurea; carbamic acid ester-based blocking agents such as phenyl N-phenylcarbaminate; imine-based blocking agents such as ethyleneimine and propyleneimine; and sulfurous acid salt-based blocking agents such as sodium bisulfite and potassium bisulfite.