Which three scopes can be used to set different System Configuration values in Magento? (Choose three.)

You are setting up a brand new Magento installation for a merchant who is migrating from Magento 1 to Magento 2.
Keeping in mind upgradability and the need to customize, which one do you choose?

In a custom module you are adding a new field to the store configuration. The field will set the value for the configuration path mycompany/mymodule/myoption.
How do you supply the default value for that configuration option?

You have created a custom module which must perform an action immediately after an order is placed, but only on the store front of the merchant site. You have selected the checkout_submit_all_after as the target event which the module will observe.
In which file will the event observer be declared?

You need to add a new text attribute to all products in the Magento store. When this attribute is displayed on the product page, its values must be different depending on the selected language.
Keeping simplicity in mind, how do you add this attribute?