Look for those records that have a field as null

Question | Sep 15, 2016 | hkumar 

Each document in a mongoDB collection - employees - has an employee's name and their optional visa type - { name, visa }. If the employee is a foreign worker the visa field is set to a string value or null. If the visa field is missing then the employee is not a foreign worker. If the visa field is set to null that means the foreign worker's visa type is not known yet.

Say the employer wants to know who are those employees (foreign workers) whose visa is not known yet.
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We can find that out by selecting only those records where visa=null. These are the sample records in collection:

{ name: "Peter Chan", visa: "TN" }
{ name: "Robert Wood" }
{ name: "Jung Woo", visa: null }

You have to tell which are all those queries from below that can select only those documents that have visa=null?