whatsapp business api is only used pre-selected company from whatsapp?
I guess whatsapp business api client is only used pre-selected company which whatsapp grant. So, i want to know my guessing is right or not.
According to official guide, i need to register phone number to whatsapp. I guess here exist whatsapp menu. but i can't see menu. (Sorry, i dont know the way website display english language)
Official guide mentions this. "If you cant see WhatsApp account option, you cant use Business API yet"
So i search how can i access Business API, and i can see this in WhatsApp FAQ. 
Finally, according to below link, WhatsApp has granted to 90 pre-selected companies. https://medium.com/callbell/what-changes-with-the-release-of-the-whatsapp-business-apis-46994316b763
As a result, if i want to use whatsapp business api, i need to use service which is provided official whatsapp partner company like twilio.
Someone tell me that My result is right?
Yes, you either need to fill out the request form to request access to WhatsApp Business API or use existing solution providers like Twilio.
do you know how much time it takes them to approve form request ?
currently, it is recommended to use one of the existing solution providers.