Is a roundtrip air ticket required for an ED visa application in Thailand, or is a one-way ticket acceptable?

Hello, I will be applying for an ED visa at the Royal Thai Consulate in Beijing, China. On their website it says to provide the following documents.

1. Guarantee letter (in English or Thai) from the applicant's local guarantor which should be sealed and attached with business card of the guarantor. The letter should state the applicant's name and purpose of the applicant's travel to Thailand.

2. A formal letter of acceptance from the educational institutions in Thailand indicating the applicant's name, duration and courses of study

3. A copy of roundtrip air ticket or an evidence of confirmed reservation

4. Course Syllabus

5. Bank statement with minimum balance of 50,000 RMB

6. Letter from Ministry of Education in Thailand * If educational institution is private school or language school, please prepare a copy of school registration and school license. Remarks: the applicants who renewed passport within one year should bring the old passport when submit application.

I'm wondering if it's actually required to show evidence of a roundtrip air ticket or if it's just okay to show a one-way ticket to Thailand.