in many parts of the world, temperature records (a)have kept/have been kept since 1860. they show that the average global temperature (b)increased/ has increased about one degree fahrenheit over the past century to more than 58 degrees fahrenheit(33 degrees celsius).the 1990s (c)was/were the warmest decade of the twentieth century.

위 문장들에서 1) 두번째 문장 해석 부탁드립니다. 2) (c)의 정답이 해답지에 were로나와 있는데요 ..1990s를 한 단위라 생각하여 was 라고 해야 하지 않을까 하는 생각이 듭니다. 왜 were를 사용해야 하는지요?