Can't figure out how to buy tanks

Lert: Every battle you earn credits and XP.   To use that XP to upgrade your tank, click on the little 'research' button to the right of the screen in the garage. You'll get a window similar to this:     The green buttons with stars are modules that haven't been researched yet. Click on them to research them. The yellow buttons are modules that have been researched but not purchased and mounted. Click on a yellow button to buy and mount the module. The checkmark signifies modules that you have mounted on your tank.   In order to buy the next tank in line, you have to research it first. In my screenshot, that is the tank called '110', that has not been researched yet. I have to grind my IS-2 to get the XP necessary to unlock the 110, before I can buy it.   (A note on mounting modules: some modules are heavier than others, and can't be mounted until the upgraded tracks have been researched and mounted)