foo.c:3:1: error: unknown type name 'bool'; did you mean '_Bool'?
 bool isBD(char c);
 ^~~~
 _Bool
foo.c: In function 'main':
foo.c:8:1: warning: 'gets' is deprecated [-Wdeprecated-declarations]
 gets(b);
 ^~~~
In file included from /usr/include/stdio.h:862:0,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.c:9:1: warning: 'gets' is deprecated [-Wdeprecated-declarations]
 gets(a);
 ^~~~
In file included from /usr/include/stdio.h:862:0,
                 from foo.c:1:
/usr/include/x86_64-linux-gnu/bits/stdio2.h:232:1: note: declared here
 gets (char *__str)
 ^~~~
foo.c: At top level:
foo.c:32:1: error: unknown type name 'bool'; did you mean '_Bool'?
 bool isBD(char c)
 ^~~~
 _Bool
foo.c: In function 'main':
foo.c:8:1: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
 gets(b);
 ^~~~~~~
foo.c:9:1: warning: ignoring return value of 'gets', declared with attribute warn_unused_result [-Wunused-result]
 gets(a);
 ^~~~~~~

[Hydro](https://hydro.ac)提供评测服务