8 USC 1158
“(B) Burden of proof
(i) In general
The burden of proof is on the applicant to establish that the applicant is a refugee, within the meaning of section 1101(a)(42)(A) of this title. To establish that the applicant is a refugee within the meaning of such section, the applicant must establish that race, religion, nationality, membership in a particular social group, or political opinion was or will be at least one central reason for persecuting the applicant.”
Here. [1]