publicclassMyLogger{privateStringBuilderlogger=newStringBuuilder（）；publicvoidlog（Stringmessage，Stringuser）{logger.append（message）；logger.append（user）；}}TheprogrammermustguaranteethatasingleMyLoggerobjectworksproperlyforamulti-threadedsystem.Howmustthiscodebechangedtobethread-safe？（）

A.synchronizethelogmethod

B.replaceStringBuilderwithStringBuffer

C.Nochangeisnecessary，thecurrentMyLoggercodeisalreadythread-safe.

D.replaceStringBuilderwithjustaStringobjectandusethestringconcatenation（+=）withinthelogmethod