查了相关的资料：发现

【可数】： An original product of human invention or artistic imagination= 指可被发明创造的事物，比如某项发明，或艺术作品等。例：The manufacturer was so proud of its creations.
【不可数】：The act of creating = 创造的过程。This company is in the creation of a number of new devices.
【不可数】：The world and all things in it = 已存在的事物。 尤指造物主的造物。Let us pray to Christ, the King of all creation.

艺术作品是可数的，可是做题的时候却发现：

Walt developed the character Mickey Mouse in 1928, his first highly popular success. He also provided the voice for his ____creation_____(create)in the early years.

答案给的是creation，但是我认为Walt在早期创作的作品肯定不止一个吧，而且根据艺术作品可数的性质，这里为什么不填 creations 呢？还是理解有误，请老师指点。