In communication ,a smile is usually __strong sign of a frie

In communication ,a smile is usually __strong sign of a friendly and open open atitude
A the, B a,an C a, D the,an
第二个空为啥不填a,换成什么词可以填?
第一个空为啥不填the?
2.（ ）panda is one of the remaining endangered animals in the world ,答案说前面空不用填,指泛指?为啥,我见过 the panda 这种用法