why can't I open the answers to my question it says upgrade but why its my first question then why isn't letting me open

Which of the choices below is one of the primary gases found in the atmosphere? a. helium b. carbon dioxide c. nitrogen d. argon

The diagram shows a series of volcanic island and a hot spot determine the direction of movement of the tectonic plate that for the island

Abookcase has a mass of 38 kg and the coefficient of friction between it and the floor is 0.82 what is the maximum force of friction between the bookcase and the floor a 372n b 305n c 412n d 449n