1. Can you really be best friends with someone of the opposite sex without ulterior motives?2. It is normal to be attracted to people of the opposite sex.
 the opposite sex the opposite gender異性(一人) = someone of the opposite sex異性(多人) = people of the opposite sex
這是一個連環問題: 燃燒石油導致二氧化碳增加，二氧化碳增加導致溫室效應，溫室效應導致冰層融化、海平面上升。 (請填空)
This is a chain of problems. Burning oil leads to increase in carbon dioxide, which leads to the greenouse effect, which leads to melting of glaciers, which leads to rising sea levels.