单项选择题
A.<if>B.<choose>C.<otherwise>D.<where>
A.<choose>、<when>、<otherwise>元素组合B.<select>、<where>、<otherwise>元素组合C.<choose>、<when>、<trim>元素组合D.<if>、<when>、<otherwise>元素组合
A.<if "username!=null and username!=’’">B.<if test="username!=null or username!=’’">C.<if test="username!=null and username!=’’">D.<trim test="username!=null and username!=’’">