How to convert 3D file (.nii) to 2D sliced .tiff file?

Question

I have a .nii file and I've tried to convert it to 2d sliced tiff file, this is what I do:

import nibabel as nib
import numpy as np
from tifffile import imsave
import tifffile as tiff

#load nii
a = nib.load('/a.nii').get_fdata()
print(np.shape(a)) #output: (512, 512, 300)

#some process need to be done(?)

#right away save the a into tiff file 
imsave('/2da.tiff', a)

at first I thought above process will work but when i load the tiffile it is still in 3d dim

tiffedA = tiff.imread('/2da.tiff')
print(tiddefA) #output: (512, 512, 300)

I expected the shape will be (300, 512, 512). I realized there is some needed process first but I don't know how, any idea to covert this 3d .nii to 2d sliced .tif in the right way?