1) the compound (I) or a prodrug thereof; 2) the compound (I) wherein X is a bond and the substituent that ring B may further have is not linked to R1; 3) the compound (I) wherein Ar is a group represented by the formula: Ar2???Ar1??? wherein Ar1 is a cyclic group optionally having substituent(s) and Ar2 is an aromatic ring group optionally having substituent(s); 4) the compound of the aforementio