About Other linker flag“-ObjC”?

Question

Description: 1. In my project(A)，I define CountryInfo class in "CountryInfoLoader.h";

@interface CountryInfo : NSObject

@property(nonatomic,strong)NSString * code;
@property(nonatomic,strong)NSString * name;
@property(nonatomic,strong)NSString * prefix;

@end

@interface CountryInfoLoader : NSObject

+ (CountryInfo*)currentCountry;
+ (NSArray*)loadAllCountries;
+ (NSDictionary*)loadGroupCountries;

@end

2. In my static library project(B, like "libB.a"), there is the same "CountryInfo.h" and "CountryInfo.m";

3. "A" dependence on "B"；

4. Some modules in “A” import "CountryInfoLoader.h" to get CountryInfo, xcode don‘t show duplicate symbol error;

5. when I add Other linker flag“-ObjC“ to fix some category error, there is duplicate symbol error on "CountryInfo"。

So： Why adding linker flag“-ObjC” causes the above error?