What did John Maynard Keynes think of aggregate demand?

a) Keynes thought the aggregate demand is stable because any decrease or increase in consumer spending is quickly matched by an equivalent increase or decrease in investment spending.
b) Keynes thought the aggregate demand is stable because people do not easily change their consumption spending and businesses always devote a significant portion of their revenue to investment purposes.
c) Keynes thought the aggregate demand is unstable because waves of pessimism and optimism can easily shift it to the right or to the left and create economic fluctuations.
d) Keynes thought the aggregate demand is unstable because long and variable policy lags worsen economic fluctuations.