Exam CRT-450 topic 1 question 259 discussion

Apex without an explicit sharing declaration is insecure by default. We strongly recommend that you always specify a sharing declaration for a class.

If a class isn’t explicitly declared as either with sharing or without sharing, the current sharing rules remain in effect. Therefore, the class doesn’t enforce sharing rules except when it acquires sharing rules from another class. For example, if the class is called by another class that has sharing enforced, then sharing is enforced for the called class. https://developer.salesforce.com/docs/atlas.en-us.apexcode.meta/apexcode/apex_classes_keywords_sharing.htm

Without Sharing is the default for an Apex class. It means you are disabling or ignoring sharing rules.

It is the default, a is wrong