diff --git a/README.md b/README.md
index 0a03cbf..25ffade 100644
--- a/README.md
+++ b/README.md
@@ -17,7 +17,7 @@ a = dot_product(u,v,coarray,option,nblock)
 
 - `coarray` is an optional logical variable. Set it to `.true.` and use the `-DUSE_COARRAY` flag to enable coarray.
 - `nblock` is an optional integer variable.
-- `options` is a character variable. Available options are `'m1'` to `'m4'`.
+- `options` is an optional character variable. Available options are `'m1'` to `'m4'`, and the default option is `'m2'`.
 
 - **Note**: Use the flag `-DUSE_DO_CONCURRENT` to enable do concurrent. This implementation of do concurrent is currently only supported by the Intel Compiler (ifx).