1. Many species of cactus whose spikes are distant relatives of the thumbtack serve as excellent accessories for an instructor's chair.[Note: assume all species have spikes]

Ans. 1. Many species of cacti, whose spikes are distant relatives of the thumbtack, serve as excellent accessories for an instructor's chair.[Note: assume all species have spikes]

1. "Cacti" is the plural of cactus

2. The clue here is that all species have spikes. Because all species have spikes, then the clause "whose spikes are distant relatives of the thumbtack" does not specify which cacti (because they all have spikes). Therefore, this clause is additional information and requires commas around it.