Please provide detailed explanation, mentioning the basics.

Which one of the following electrocyclic reactions should occur readily by a concerted mechanism? (concerted due to disrotatory ring closure) Explain why two different products are formed from the disrotatory ring closure of (2E,4Z,6Z)-octatriene, but only one product is formed from disrotatory ring closure of (2E,4Z,6E)-octatriene. (2Z,4Z,6E)-octa-2,4,6-triene (2E,4Z,6E)-octa-2,4,6-triene