我下载了一些netCDF数据，并且只请求了纬度，经度和时间。但是，我得到的.nc文件似乎有四个尺寸，包括一个空的海拔尺寸（请参见下文）。如何完全删除此尺寸，以便nc对象只有三个尺寸（在R中）？谢谢！
> print(nc)
File (NC_FORMAT_CLASSIC):
1 variables (excluding dimension variables):
float sst[longitude,latitude,altitude,time]
_ChunkSizes: 1
_ChunkSizes: 1
_ChunkSizes: 720
_ChunkSizes: 1440
colorBarMaximum: 32
colorBarMinimum: 0
coordinates: time_run time zlev lat lon
ioos_category: Temperature
long_name: Daily Sea Surface Temperature
standard_name: sea_surface_temperature
units: degree_C
4 dimensions:
time Size:366
_CoordinateAxisType: Time
actual_range: 378691200
actual_range: 410227200
axis: T
calendar: proleptic_gregorian
ioos_category: Time
long_name: Center Time
missing_value: NaN
standard_name: time
time_origin: 01-JAN-1970 00:00:00
units: seconds since 1970-01-01T00:00:00Z
altitude Size:1
_CoordinateAxisType: Height
_CoordinateZisPositive: up
actual_range: 0
actual_range: 0
axis: Z
colorBarMaximum: 10
colorBarMinimum: -10
ioos_category: Location
long_name: Altitude
positive: up
standard_name: altitude
units: m
latitude Size:57
_CoordinateAxisType: Lat
actual_range: 34.125
actual_range: 48.125
axis: Y
ioos_category: Location
long_name: Latitude
standard_name: latitude
units: degrees_north
longitude Size:73
_CoordinateAxisType: Lon
actual_range: -77.875
actual_range: -59.875
axis: X
ioos_category: Location
long_name: Longitude
standard_name: longitude
units: degrees_east