The correct sentences would be:
1. Las estudiantes de biología están nerviosas porque los cursos son difíciles.
In Spanish, the verbs "ser" and "estar" both mean "to be," but they are used in different contexts. "Ser" is used to express permanent or inherent qualities, while "estar" is used to express temporary states or conditions. In the given sentence, "los cursos son difíciles" refers to a permanent quality or characteristic of the courses, so we use "ser." On the other hand, "las estudiantes están nerviosas" describes a temporary state or condition, so we use "estar" to express that they are currently feeling nervous.